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Irrigation efficiency and water-saving potential considering reuse of return flow
Agricultural Water Management 221 (2019) 519–527
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Irrigation efficiency and water-saving potential considering reuse of return flow
T
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Di Wu, Yuanlai Cui , Yufeng Luo State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, 430072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Irrigation efficiency Water-saving potential Modified SWAT Reuse of return flow
Irrigation efficiency (IE) and water-saving potential (WSP) are two fundamental parameters for assessing water use and management in irrigation systems. A new calculation method was proposed herein to accurately estimate the IE and WSP in irrigation systems. The proposed method considers the reuse of return flow. A modified Soil and Water Assessment Tool (SWAT) was used to simulate hydrological processes under various water-saving scenarios for the Yangshudang (YSD) watershed within the Zhanghe Irrigation System (ZIS) in Hubei Province, China. The dry year of 2010 was chosen as a study case. Based on simulation results, the traditional irrigation efficiency (IE0 ) and water-saving potential (WSP0 ) as well as the irrigation efficiency taking into account the reuse of return flow (IEr ) and water-saving potential considering the reuse of return flow (WSPr ) were calculated for various scenarios. The relationships between the two IE indicators and the cause thereof, as well as the two WSP values, were analyzed and explored. The results showed that both IE and WSP were improved with the enhancement of water saving. As long as there was the reuse of return flow, IEr must be greater than IE0 . Moreover, in terms of water-saving approaches that improved the reuse rate of return flow, WSPr was determined to be greater than WSP0 , thereby suggesting that the traditional method underestimated the WSP. However, for water-saving approaches that reduced the reuse rate of return flow, WSPr was determined to be less than WSP0 , which suggested that the traditional method overestimated the WSP. The relationship between WSP0 and WSPr was attributed to the fact that WSPr was calculated by subtracting the amount of the water saved by the reuse of return flow on the basis of WSP0 , and this difference can be either positive or negative. Therefore, the managers of irrigation systems should use IEr as the actual IE but not IE0 , and use WSPr instead of WSP0 to evaluate the actual WSP.
1. Introduction Irrigation efficiency (IE) is an indicator that comprehensively reflects the efficiencies of irrigation projects, water management, and the irrigation technology at various scales. It is an important basis for correctly evaluating the effective use degree of irrigation water and the development effect of water-saving irrigation. Water-saving potential (WSP) refers to the amount of water that can be conserved in an irrigation system by adopting one or more water-saving approaches under certain socio-economic conditions. The correct calculation of WSP is of significance for guiding water-saving reforming engineering in irrigation systems. Presently, the effective use degree of irrigation water is evaluated by using the traditional IE (IE0 ). The difference between the gross irrigation water use before and after the application of water-saving approaches is used to describe the WSP, which is calculated based on the
⁎
net irrigation requirement and IE0 . This value is also known as the water-saving amount for irrigation water intake (WSP0 ) (Cui and Xiong, 2009; Cui et al., 2014). However, IE0 is calculated without considering the reuse of the return flow in irrigation systems; therefore, it cannot reflect the true effective use degree of irrigation water and the level of water management in irrigation systems. Moreover, WSP0 is not the true amount of water saved, because the amount of water leaking into the deep ground and the amount returning to the surface are also considered to be savable in the calculation of WSP0 . However, in reality, a part of these return flows still remains in the water resource system and can be reused. Therefore, this portion of return flows should not be counted as savable. Seckler (1996) indicated that water loss at the field scale might not amount to real loss because it may be reused on the larger scale. He further indicated that misconceptions regarding IE can lead to erroneous estimation of WSP. Solomon and Davidoff (1999) pointed out that the relationship between IE at the field scale and IE at
Corresponding author. E-mail address: [email protected] (Y. Cui).
https://doi.org/10.1016/j.agwat.2019.05.021 Received 1 March 2019; Received in revised form 17 May 2019; Accepted 18 May 2019 Available online 23 May 2019 0378-3774/ © 2019 Elsevier B.V. All rights reserved.
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managers. 2 To study the YSD watershed in the ZIS in Hubei Province, China, as a case. We applied the new method to calculate the IEr and WSPr of YSD under various water-saving scenarios. Additionally, we calculated IE0 and WSP0 by using traditional methods. 3 To analyze the change rules of IE0 , IEr , WSP0 and WSPr under various water-saving scenarios, and the relationships between them and the causes thereof.
the irrigation system or basin scale was not well understood, and the complex relationship between these scales was attributed to the fact that the drainage water of one farmland may be used by another farmland, i.e., there was the reuse of return flow. Li et al. (2005) and Cui et al. (2007) analyzed the different WSPs in the Zhanghe Irrigation System (ZIS) in Hubei Province, China, and determined that the WSP calculated based on water balance was much smaller than that calculated based on the IE0 and net irrigation requirement. Blanke et al. (2007) indicated that the adoption of water-saving engineering methods and technology could be effective on a relatively small scale but was extremely costly and failed to save water because a vast majority of “losses” were reused. Therefore, it is important to explore new calculation methods for IE and WSP by taking the reuse of return flow into account. Herein, IEr is the irrigation efficiency with the reuse of return flow and WSPr is the water-saving potential with reuse of the return flow. Considering these findings, many researchers have proposed new methods for calculating IE and WSP by considering the reuse of the return flow from the perspective of water resource management in irrigation systems (Jensen, 1977; Keller and Keller, 1995; Molden, 1997; Campadelli et al., 2007; Cui et al., 2010; Liu et al., 2011, 2013). Although the rationality of the associated concepts and theories have been emphasized in these methods, the application of these methods in real situations has been challenging owing to the difficulties associated with determining some of the water balance elements, especially the return flow and its reused amount. Such difficulties have hindered the application of these methods to water management in irrigation systems. Therefore, the difficulty in correctly evaluating the level of water management and water-saving potential in the irrigation systems lies in proposing practical methods to estimate IE which consider the reuse of return flow and quantifying the return flow and its reused amount. Recently, many studies have explored the process of quantifying the return flow and its reused amount (Zulu et al., 1996; Mohan and Vijayalakshmi, 2009; Chien and Fang, 2012; Ha et al., 2017). However, most of these studies only emphasized the estimation of the quantity of return flow, with limited consideration of the quantification of the reuse of the return flow. Apart from these difficulties, it is too difficult and expensive to conduct experiments for calculating the return flow and its reused amount over large-scale areas such as irrigation systems. Therefore, modeling methods are suitable for these cases that involve large-scale areas. Considering the spatial heterogeneity of irrigation systems, a distributed hydrological model is the best choice for the estimation of the return flow and its reused amount. Consequently, the development of a distributed hydrological model capable of describing hydrological processes in an irrigation system is the key to estimating the return flow and its reused amount. Wu et al. (2019a; 2019b) modified the Soil and Water Assessment Tool (SWAT) (Arnold et al., 1998; Neitsch et al., 2011) and proposed a new method for calculating the return flow and its reused amount in irrigation systems. Their method was based on the simulation results obtained from the modified SWAT model. These studies notwithstanding, there is little research on the relationship between WSPr and WSP0 . From the perspective of irrigation water management, the managers of irrigation systems hope to obtain the accurate IE and WSP. But at present, IE0 and WSP0 do not take the reuse of return flow into account, where IE0 is underestimated and WSP0 cannot represent the actual WSP, which may lead the managers to make improper water-saving measures and water management decisions. IEr and WSPr take the reuse of return flow into account and can be used to evaluate actual WSP, which can provide irrigation management decision support for the managers, while the current methods proposed by predecessors to calculate them are difficult to be used in practice. From what has been discussed above, the objectives of this study are as follows:
2. Materials and methods 2.1. Irrigation efficiency considering reuse of return flow 2.1.1. Definitions of indicators A few definitions of the irrigation efficiency from different perspectives (Israelsen, 1950; Jensen, 1977; Guo, 1997; Liu et al., 2011; Huffman et al., 2013; Cui et al., 2014) have been proposed during the past few decades. IE0 is the most widely used and is defined as follows:
IE0 =
Ws Wg
(1)
where Ws is the water applied to the soil root zone through irrigation (m3) and Wg is the gross irrigation water use, i.e., the sum of water diverted from all different water sources for irrigation (m3) (Guo, 1997; Huffman et al., 2013). As shown in Eq. (1), IE0 does not consider the impact of the reuse of return flow. Nevertheless, the return flow and its reuse are common phenomena in any irrigation system. To accurately reflect the irrigation water use efficiency, assess the level of water management, and calculate the WSP in an irrigation system, in this study, IEr is defined as follows:
IEr =
WI W + Wru = s Wg Wg
(2)
where WI is the water applied to the soil root zone through irrigation and the reuse of return flow (m3) and Wru is the water applied to the soil root zone through the reuse of return flow (m3). IEr considers the impact of the reuse of return flow not only from the water use terminal, i.e., from the soil root zone, but also from the water intake terminal, i.e., from the water sources. Therefore, IEr can also be defined as follows:
IEr =
Ws Ws = Wnew, g Wg − Wr
(3)
where Wnew, g is the gross irrigation water use that does not include the reused return flow (m3), which is equal to Wg minus the gross reused return flow in the irrigation system; Wr is the gross reused irrigation return flow (m3), which is the return flow taken from drainage channels or ponds for irrigation. In terms of the definition of IEr , as shown in Eq. (2), the numerator and denominator simultaneously contain the reused return flow, the numerator contains the net, and the denominator contains the gross. While as shown in Eq. (3), the numerator and denominator do not contain the reused return flow. In other words, the definition of IEr needs that both the numerator and denominator involve the reuse of return flow, or neither. Compared with IE0 , the calculation of IEr considers the reuse of return flow for irrigation by adding the water applied to the soil root zone through the reuse of return flow in the numerator or by subtracting the gross reused return flow in the denominator. To explore the relationship between IE0 and IEr , Eq. (2) is further represented as
WI W + Wru W W = s = s + ru Wg Wg Wg Wg Wr Wru = IE0 + ⋅ = IE0 + ηI ⋅ηr Wg Wr IEr =
1 To define and propose a practical method for calculating IEr and WSPr for providing irrigation management decision support for the 520
(4)
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Fig. 1. Study area and sub-basins division.
where ηr is the use rate of the irrigation return flow defined as the percentage of Wru in Wr ; ηI is the irrigation water reuse rate defined as the percentage of Wr in Wg . Eq. (3) can be further expanded on the basis of the equal ratios theorem in mathematics as follows:
2.2. Methods for calculating the water-saving potential In this study, the focus is on the analysis of WSP0 and WSPr . The equation for calculating WSP0 is as follows (Cui and Liu, 2015):
W W WSP0 = ⎛ n ⎞ − ⎛ n ⎞ ⎝ IE0 ⎠b ⎝ IE0 ⎠a ⎜
Ws +
Ws ⋅Wr Wg − Wr
Ws Ws Ws + IEr⋅Wr IEr = = = = Wnew, g Wg − Wr Wg − Wr + Wr Wg − Wr + Wr W IE ⋅W W = s + r r = IE0 + IEr⋅ r = IE0 + IEr⋅ηI Wg Wg Wg
IE0 1 − ηI
⎜
⎟
(7)
where Wn is the net irrigation requirement, which is generally obtained by multiplying the net irrigation quota by the irrigated area, (m3), and it is different from Ws that refers to the single use of irrigation water and excludes the reuse of return flow, whereas Wn includes the single use of irrigation water and the reuse of return flow, which corresponds to the sum of Ws and Wru ; the subscripts b and a represent before and after the application of the water-saving approaches. To calculate the WSP that accounts for the reuse of return flow, it is necessary to replace IE0 by IEr on the basis of Eq. (7):
(5)
2.1.2. Simplification of indicators As shown in Eq. (4), the calculation of IEr requires that IE0 , ηI , and ηr should be known. IE0 can be calculated by monitoring the amount of water introduced at the head of the canal and the amount of water applied to the soil root zone. Alternatively, it can be obtained from the management department of the irrigation system. ηI can be calculated by using the method proposed by Wu et al. (2019a) based on the modified SWAT model. But it is difficult to measure or estimate the value of ηr . Here it is assumed that the effective use degree of Wr is the same as that of Wg , that is to say, ηr = IEr . The equation for calculating IEr can be simplified by substituting this relationship into Eq. (4):
IEr =
⎟
W W WSPr = ⎛ n ⎞ − ⎛ n ⎞ ⎝ IEr ⎠b ⎝ IEr ⎠a ⎜
⎟
⎜
⎟
(8)
It can be seen that the main difference between the two WSPs is attributed to the difference in the methods for calculating the IE. The method used in Eq. (7) is referred to as the traditional method. The method used in Eq. (8) accounts for the reuse of the return flow. Therefore, the WSP calculated using Eq. (8) considers the change of the reused return flow after applying the water-saving approaches and can better reflect the true water-saving potential in an irrigation system.
(6)
2.3. Study area and model setup
As for Eq. (5), it can be transformed into Eq. (6) by equation transformation. This demonstrates that the final equations for calculating the irrigation efficiency are the same when considering the reuse of return flow from the two different perspectives. Furthermore, it can be seen from Eq. (6) that IEr is always greater than IE0 as long as that there is the reuse of the return flow.
2.3.1. Study area The study area was the YSD watershed in the ZIS of the Hubei Province in China. It is a relatively closed area of approximately 43.3 km2. The area is enclosed by the third main canal of the ZIS, the 521
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Table 1 Critical water depths at different growing stages of paddy rice for various rice irrigation modes. Item
Beginning date Ending date Flood irrigation Intermittent irrigation "Thin-shallow-wet-dry"
Steeping stage
May 20 May 29 20-40-80 20-40-80 10-20-80
Recovering stage
May 30 Jun 7 10-40-60 10-30-50 5-20-50
Tillering stage
Booting stage
Wet
Dry
Jun 8 Jul 5 10-40-60 10-40-60 0-25-60
Jul 6 Jul 14 20-60-80 0-0-0 0-0-0
Jul 15 Jul 30 20-60-80 20-50-70 10-35-70
Heading stage
Jul 31 Aug 9 20-60-80 20-50-70 5-35-70
Milky stage
Aug 10 Aug 18 20-60-80 10-40-60 0-25-60
Ripening stage Wet
Dry
Aug 19 Aug 26 20-60-80 0-20-30 0-10-25
Aug 27 Sep 3 0-0-0 0-0-0 0-0-0
Note: The critical water depths for irrigation modes are in the form of “Hmin − Hmax − Hp ” and the unit is millimeter.
reservoir to the fields. Therefore, two different traditional IE values were set for the different water sources. Under the present scenario, these values are IE01 = 0.90 for the local water sources and IE02 = 0.65 for the Zhanghe reservoir. The overall traditional IE is calculated as the weighted average based on the corresponding irrigation water use of the two water sources. In particular, the traditional IE associated with the Zhanghe reservoir can be increased by improving the anti-seep standards of the corresponding canals. Based on the present scenario (IE02 is 0.65), we gradually increased the traditional IE of Zhanghe reservoir by 0.05. In other words, IE02 was set to be 0.70, 0.75, 0.80, and 0.85. 3 Different rice irrigation modes. Because the study area is a typical irrigation system with paddy rice, three different irrigation modes were set for the irrigation system. These included the flood irrigation mode, intermittent irrigation mode, and the “thin-shallow-wetdry” irrigation mode. The three critical water depths of paddy fields at different growing stages for these three irrigation modes, namely, the maximum fitting depth (Hmax ), the minimum fitting depth (Hmin ), and the maximum allowable depth of impoundment after rainfall (Hp ), are shown in Table 1. 4 A comprehensive water-saving scenario. For each of the abovementioned three scenarios, there exists a specific condition where the water-saving potential is maximized. Therefore, integrating the conditions with the maximum water-saving potential associated with the three different scenarios will yield a comprehensive watersaving scenario.
first tributary canal of the third main canal, and a small branch canal, as shown in Fig. 1 (Wu et al., 2019a). The study area has a subtropical continental climate, with an annual average temperature of 17 °C, maximum temperature of 40.9 °C, and an annual mean rainfall of 821 mm. The mainland includes paddy fields, dry lands, forests, and grasslands, of which the paddy fields account for approximately 60%. The soil type is predominantly yellow-brown earth paddy soil. The study area cultivates crops such as rice, cotton, and rapeseed. Rice is the main irrigated crop and uses the intermittent irrigation mode. 2.3.2. Model setup According to Eq. (8), Wn and IEr must be determined prior to calculating WSPr . However, Eq. (6) implies that ηI must be obtained before calculating IEr . For the whole irrigation system, ηI can be calculated by using the method proposed by Wu et al. (2019a), which was based on the modified SWAT model (Wu et al., 2019b). The modified SWAT model allows for the simulation of hydrological processes under various water-saving scenarios. In this study, a distributed hydrological model was set up for the study area based on the modified SWAT model and the ηI was calculated using the method proposed by Wu et al. (2019a). The study area was divided into 10 sub-basins based on the digital elevation model (DEM) data as shown in Fig. 1. In particular, in terms of the YSD watershed, the river channels as shown in Fig. 1 are mainly used as the actual drainage channels. Furthermore, 105 hydrologic response units (HRUs) were obtained according to the type of land use, soil type distribution, and slope division. Next, a distributed hydrological model was established for the YSD watershed based on meteorological data and relevant parameters of ponds. In addition, the model was calibrated and validated using the observed discharge series in the YSD watershed outlet, the observed evapotranspiration series, and the irrigation water use from different water sources measured in the study area. For the detailed modeling, calibration, and validation process and the associated results one can refer to the study by Wu et al. (2019b).
3. Results and discussion 3.1. Results and analysis of irrigation efficiency and water-saving potential The hydrological processes under the present scenario and various water-saving scenarios described in the preceding sections were simulated with the modified SWAT model of the study area. We ranked the rainfall between 1973 and 2010 from large to small during the paddy rice growth period in the study area to calculate the rainfall frequency. We selected the dry year of 2010 (rainfall frequency is 90%) for the case study. Given that the difference between the new method proposed in this study and the traditional method is the consideration of the reuse of the return flow, ηI should be calculated under various water-saving scenarios according to the simulation results. Moreover, the corresponding values of IE0 and IEr were also calculated according to the simulation results, where IE0 was calculated as the weighted average based on irrigation water use from the local water sources and the Zhanghe reservoir, as well as the corresponding IE01 and IE02 values. IEr was calculated directly using Eq. (6). Finally, WSP0 and WSPr were calculated using Eqs. (7) and (8), respectively.
2.4. Setting up water-saving scenarios To explore the change rules of WSP0 and WSPr under various watersaving scenarios, the relationship between the two water-saving potentials, as well as the underlying causes of this relationship, four different water-saving scenarios were set up based on the characteristics of the study area: 1 Different drainage areas of ponds. The change in the drainage areas of ponds was represented by the amplification in a fraction of the sub-basin area that drains into ponds ( pnd _fr ). There is a pnd _fr for each sub-basin. Based on the present scenario (the amplification is 0%), the amplifications were set as 10%, 20%, 30%, 40%, and 50%. 2 Different traditional irrigation efficiencies. As far as the YSD watershed is concerned, the main water sources include both the local water sources (ponds and drainage channels) and the Zhanghe reservoir. The canal system is rarely used for the local water sources. However, it is required for transporting water from the Zhanghe
3.1.1. Results and analysis under various drainage areas of ponds According to the simulation results with the modified SWAT for YSD under various drainage areas of ponds, the corresponding values of ηI , IE0 , IEr , WSP0 , and WSPr were calculated as shown in Fig. 2. The following conclusions can be derived from Fig. 2: 522
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Fig. 2. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various drainage areas of ponds.
3.1.2. Results and analysis under various traditional irrigation efficiencies According to the simulation results with the modified SWAT for YSD under various IE02 values (traditional IE for water from the Zhanghe reservoir), the corresponding values of ηI , IE0 , IEr , WSP0 , and WSPr are shown in Fig. 3. The following conclusions can be derived from Fig. 3:
1 ηI increased gradually with an increase in the drainage areas of ponds because of an increase in the amount of return flow captured by the ponds with an increase in the drainage area of ponds. The increase in the return flow further resulted in an increase in the amount of the reused irrigation return flow. Consequently, increasing the drainage area of ponds could be classified as a watersaving approach that improved the reuse rate of return flow. 2 Both IE0 and IEr increased gradually with an increase in the drainage areas of ponds. This was attributed to the fact that the total amount of irrigation water required for the crops remained unchanged. Therefore, with an increase in the drainage areas of ponds, more water can be captured by the ponds. This causes an increase in the amount of irrigation water from local water sources. Correspondingly, less irrigation water was required from the Zhanghe reservoir. In addition, because the IE of the local water sources was higher than that of the Zhanghe reservoir, the overall IE was improved. IEr was determined to be greater than IE0 , that was because according to Eq. (6), as long as there was the reuse of return flow, i.e., ηI > 0, IEr was greater than IE0 . 3 Both WSP0 and WSPr increased gradually with an increase in the drainage areas of ponds. According to Eqs. (7) and (8), increases in IE0 and IEr will inevitably lead to increases in WSP0 and WSPr . In addition, WSP0 was determined to be smaller than WSPr under the water-saving scenario with an increase in the drainage areas of ponds. This indicated that the traditional method underestimated the real water-saving potential under this scenario.
1 ηI decreased gradually with an increase in IE02 . This decrease was caused by the reduction in the water conveyance losses in the canal system with an increase in IE02 , which led to a reduction in the return flow. ηI became smaller because less irrigation return flow was reused in the field. Based on this trend, an increase in IE02 could be classified as a water-saving approach that decreased the reuse rate of return flow. 2 Both IE0 and IEr increased with an increase in IE02 . Because the total amount of net irrigation requirement for the crops remained unchanged, the reduction in the water conveyance losses in the canal system from the Zhanghe reservoir to fields resulted in a smaller gross irrigation water use. This further led to a higher irrigation efficiency. In addition, Fig. 3 showed that IEr was greater than IE0 because of the reuse of return flow. 3 Both WSP0 and WSPr increased gradually with an increase in IE02 . This trend was caused by increases in IE0 and IEr as previously explained. In addition, WSP0 was determined to be greater than WSPr under the water-saving scenario with an increase in the traditional IE. This result indicated that the traditional method overestimated the real water-saving potential in this scenario.
The above results showed that increasing the drainage areas of ponds could enhance the reuse of return flow, increase the IE values and save water for irrigation. In order to increase the drainage areas of ponds in irrigation systems, the managers of irrigation systems can take some measures, such as constructing ponds and digging small ditches to divert return flow into ponds. And the managers should decide whether or not to adopt these measures in real-practice irrigation systems according to the amplification in IEr and the size of WSPr , instead of IE0 and WSP0 under the traditional method.
As mentioned above, it can be seen that an increase in IE02 could increase the overall IE values and save water for irrigation. Some measures can be taken to improve the anti-seep standards of the corresponding canals for increasing IE02 , such as lining irrigation canals, and if the investment is enough, using water pipelines instead of canals. While increasing IE02 would cut down on the reuse rate of return flow, the managers of irrigation systems do not have to increase IE02 much, because the existence of the reuse of return flow can increase IEr together with the increase of IE02 .
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Fig. 3. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various values of IE02 .
water-saving irrigation mode compared with the traditional flood irrigation mode. Therefore, water-saving potentials can be generated by replacing the traditional flood irrigation mode with water-saving irrigation modes. Moreover, ηI was determined to be higher when the intermittent irrigation mode was used compared with the “thin-shallowwet-dry” irrigation mode. However, the WSP0 and WSPr of the intermittent irrigation mode were smaller than those of the “thin-shallowwet-dry” irrigation mode. This is because replacing the flood irrigation mode with the water-saving irrigation mode not only improved the ηI value but also reduced the net irrigation water use required for the paddy rice. Furthermore, the application of the “thin-shallow-wet-dry” irrigation mode could reduce the net irrigation water requirement to a
3.1.3. Results and analysis under various irrigation modes During the analysis of the results under various irrigation modes, the flood irrigation mode was used as the basis for calculating the water-saving potentials of the two water-saving irrigation modes. Fig. 4 shows the values of ηI , IE0 , IEr , WSP0 , and WSPr under various irrigation modes. As shown in Fig. 4, the ηI values under two water-saving irrigation modes, namely, the intermittent irrigation mode and “thin-shallow-wetdry” irrigation mode, were higher than that under the flood irrigation mode. This result indicated that water-saving irrigation could be classified as a water-saving approach that increased the reuse rate of return flow. In addition, both IE0 and IEr were found to be higher under the
Fig. 4. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various irrigation modes. 524
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corresponding water amounts and indicators of NO. 3 sub-basin were calculated as shown in Table 3. The results showed that with the increase of drainage areas of ponds, the reused irrigation return flow increased, which made the gross irrigation water use decrease and ηI increase. In addition, the IEs and WSPs increased, and IEr was greater than IE0 and WSPr was greater than WSP0 . Combining the results and analysis on the whole YSD watershed scale, it could be found that the change rules of indicators and relationships between them in the NO. 3 sub-basin were same as that in the whole YSD watershed. That is to say, the results are correct and universally applicable for the scenarios of increasing drainage areas of ponds, and similarly, in other scenarios, the results on the sub-basin scale and whole watershed scale are consistent.
greater extent, and therefore yield a larger water-saving potential compared with the case where the intermittent irrigation mode was used. Fig. 4 showed that WSP0 was smaller than WSPr when the watersaving irrigation modes were used. In other words, the traditional method underestimated the real water-saving potential under this scenario. The above analysis indicated that the rice water-saving irrigation modes could not only improve the reuse rate of return flow and IE values, but also save water. The managers of irrigation systems can carry out the rice water-saving irrigation modes in the irrigation systems with paddy rice. Besides, some water-saving irrigation technologies like drip and sprinkle can be used for upland crops, the micro irrigation systems save water mainly by reducing the loss of water delivery system and field irrigation requirement, so the multi-source auto-irrigation module of the modified SWAT model can be used to reflect this situation, which is through increasing the traditional IE of water sources and decreasing irrigation water amount in an irrigation application, further the hydrological processes under corresponding scenarios can be simulated and the ηI , IEr and WSPr can be calculated, which can assist them to decide whether to adopt these technologies or not.
3.2. Relationship between the two water-saving potentials and cause thereof Based on the WSP0 , WSPr , and ηI values obtained under various water-saving scenarios in Section 3.1, it can be concluded that: 1 Increasing the drainage areas of ponds and using a water-saving irrigation mode for rice were the water-saving approaches that increased the reuse rate of the return flow. For these approaches, WSP0 was smaller than WSPr , suggesting that the traditional method underestimated the real water-saving potential; 2 Increasing IE02 and integrating multiple water-saving methods were water-saving approaches that reduced the reuse rate of the return flow. For these approaches, WSP0 was greater than WSPr , which suggested that the traditional method overestimated the watersaving potential.
3.1.4. Results and analysis under the comprehensive water-saving scenario As shown in Figs. 2–4, the individual water-saving scenarios with the highest water-saving potentials include: (1) the amplification of 50% in the drainage areas of ponds, (2) IE02 = 0.85, and (3) the “thinshallow-wet-dry” irrigation mode. Therefore, a comprehensive watersaving scenario can be constructed by combining these three watersaving scenarios. The values of ηI , IE0 , IEr , WSP0 , and WSPr for the comprehensive water-saving scenario were calculated based on the corresponding simulation results with the modified SWAT model and are listed in Table 2. As shown in Table 2, ηI became smaller under the comprehensive water-saving scenario compared with the present scenario. In other words, the comprehensive water-saving scenario could be classified as a water-saving approach that reduced the reuse rate of return flow. Both IE0 and IEr were determined to be higher under the comprehensive water-saving scenario compared with the values obtained under the present scenario. This indicated that there is high WSP under comprehensive water-saving scenarios compared with the present scenario. As shown in Table 2, WSP0 was greater than WSPr . This suggested that the traditional method overestimated the water-saving potential under the comprehensive water-saving scenario. Furthermore, when comparing the WSP given in Table 2 with that obtained under the individual watersaving scenarios, it was determined that combining multiple watersaving scenarios could yield a higher WSP. Therefore, combining multiple water-saving scenarios is most effective in reducing irrigation water use.
In addition, as shown in Fig. 4, the “thin-shallow-wet-dry” irrigation mode exhibited greater WSP than the intermittent irrigation mode. The WSP0 and WSPr of the “thin-shallow-wet-dry” irrigation mode calculated on the basis of the intermittent irrigation mode were 1,054,900 m3 and 872,600 m3, respectively. The ηI of the “thin-shallow-wet-dry” irrigation mode was smaller than that of the intermittent irrigation mode. This also suggested that WSP0 was greater than WSPr for the watersaving approach that reduced the reuse rate of the return flow, which was consistent with the previous conclusion. Furthermore, a similar conclusion was also drawn by Cui et al. (2014) when analyzing the relationship between the two WSPs in the Liuyuankou Irrigation System in north China. Therefore, the above conclusions might be universal. To further analyze the cause for the previously discussed relationship between WSP0 and WSPr , we substituted Eq. (6) into Eq. (8) and performed an additional analysis of the equation for calculating WSPr as follows:
W W WSPr = ⎛ n ⎞ − ⎛ n ⎞ IE r ⎝ ⎠b ⎝ IEr ⎠a W W = ⎡ n (1 − ηI ) ⎤ − ⎡ n (1 − ηI ) ⎤ ⎥ ⎥ ⎢ ⎦a ⎣ IE0 ⎦b ⎢ ⎣ IE0 ⎜
3.1.5. Results and analysis on the sub-basin scale The abovementioned results were mainly on the whole watershed scale, the results on the sub-basin scale were also needed to be analyzed. Wu et al. (2019b) verified the water balance components of NO. 3 sub-basin (Fig. 1), so it was taken as a typical sub-basin. Taking the scenarios of increasing drainage areas of ponds as examples, and according to the simulation results with modified SWAT model, the
ηI /(%)
IE0
IEr
WSP0/(10 4m3)
WSPr /(10 4m3)
Present condition Comprehensive watersaving scenario
8.02% 7.20%
0.72 0.73
0.78 0.79
/ 347.43
/ 310.41
⎜
⎟
⎛ Wn ⎞ ⎤ ⎛ Wn ⎞ ⎛ Wn ⎞ ⎤ ⎡ ⎛ Wn ⎞ =⎡ ⎢ IE0 − IE0 ⎥ − ⎢ IE0 ηI − IE0 ηI ⎥ ⎠b ⎝ ⎠a ⎦ ⎠b ⎝ ⎠a ⎦ ⎣ ⎝ ⎣⎝ ⎜
⎟
⎜
⎟
⎜
⎟
⎛ Wn ⎞ ⎛ Wn ⎞ ⎤ = WSP0 − ⎡ ⎢ IE0 ηI − IE0 ηI ⎥ ⎝ ⎠ ⎝ ⎠a ⎦ b ⎣ ⎜
Table 2 Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under the comprehensive water-saving scenario. Water-saving scenario
⎟
⎟
⎜
⎜
⎟
⎟
(9)
As shown in Eq. (9), WSPr is calculated by subtracting the water-saving amount difference induced by the reuse of return flow on the basis of WSP0 . For water-saving approaches that maintain a constant Wn such as increasing the drainage area of ponds or increasing the traditional IE, the water-saving amount difference is a positive if ηI decreases after adopting the water-saving approaches. In this case, WSP0 is smaller than WSPr . But if ηI increases, the result will be the opposite. Moreover, for water-saving approaches that reduce Wn such as changing the irrigation 525
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Table 3 The simulation and calculation results of NO.3 sun-basin under the various drainage areas of ponds. Amplification in pnd _fr (%)
Wg /(10 4m3)
Wr /(10 4m3)
ηI /(%)
IE0
IEr
WSP0/(10 4m3)
WSPr /(10 4m3)
0 10 20 30 40 50
181.94 180.77 179.79 179.08 178.37 177.80
11.99 12.12 12.32 12.50 12.67 12.83
6.59% 6.71% 6.85% 6.98% 7.11% 7.22%
0.72 0.72 0.73 0.73 0.73 0.73
0.77 0.77 0.78 0.78 0.79 0.79
0.00 1.17 2.15 2.86 3.57 4.13
0.00 1.30 2.48 3.37 4.26 4.97
efficiency compared with the flood irrigation mode; (4) the two WSPs of a comprehensive water-saving scenario were greater than those of a single water-saving scenario. A comparison showed that IEr was greater than IE0 provided that the reused return flow was nonzero. A comparison and analysis of WSP0 and WSPr indicated that for the water-saving approaches that improved the reuse rate of the return flow, such as increasing the drainage area of ponds or using a water-saving irrigation mode for rice, the traditional method underestimated the water-saving potential. For the watersaving approaches that reduced the reuse rate of the return flow, such as increasing the traditional irrigation efficiency, the traditional method overestimated the water-saving potential. This was because the difference of water-saving amount induced by the reuse of the return flow was included in the calculation for WSPr on the basis of WSP0 , and the difference can be either positive or negative. In addition, the abovementioned change rules of the indicators and relationships between them in a sub-basin are same as that in the whole watershed, which means that the results are correct and universal. Consequently, this study systematically analyzed the relationship between IE0 and IEr as well as that between WSP0 and WSPr , for the sake of accuracy, the managers of irrigation systems should use IEr as the IE of the irrigation systems but not IE0 , and use WSPr instead of WSP0 to evaluate the actual WSP after adopting one or more water-saving approaches. Moreover, further researches are needed for better using these indicators in reality, such as simplifying the calculation of intermediate variables, etc.
mode for rice, the water-saving difference is determined by both Wn and ηI . However, the magnitude of the reduction in Wn is usually quite small to ensure that the irrigation requirements of crops can be satisfied. Consequently, the sign of the water-saving amount difference is primarily determined by ηI . According to Eq. (9) and the above description, the previously shown relationship between WSP0 and WSPr is caused by the fact that WSPr is calculated by incorporating a watersaving amount difference term induced by the reuse of return flow on the basis of WSP0 . The traditional IE can usually be obtained from the management of the irrigation systems. In this study, we proposed a new method to calculate IEr and WSPr by accounting for the reuse of the return flow. By combining the modified SWAT model proposed by Wu et al. (2019b) for multi-source irrigation systems and the calculation method for ηI based on the simulation results with the modified SWAT model (Wu et al., 2019a), we established a comprehensive set of methods for calculating IEr and WSPr .The proposed method can be used to accurately reflect the level of irrigation water management and evaluate the WSP in an irrigation system. In addition, the proposed method can be effectively used in real-practice irrigation systems. Wang et al. (2017) used Bayesian multi-model to project the water use efficiency of rice plantation region owing to the climate change, meanwhile, the modified SWAT model and the method in this study also can be used to project the IEr and WSPr under the climate change, which can provide decision support for irrigation system managers to prevent negative effects caused by climate change and guide the direction of water-saving reforming engineering. As shown in Eq. (6), if the value of ηI in an irrigation system can be obtained, IEr can be calculated based on IE0 . Although the calculation method for ηI proposed by Wu et al. (2019a) can be used in most irrigation systems, it is difficult to use in the irrigation systems where detailed data is lacking. Therefore, we can calculate the ηI of multiple irrigation systems and give the range of ηI , or further establish a statistical model between ηI and the characteristic parameters of irrigation systems, such as terrain parameters, crops planting structure, meteorological parameters and so on. In practical application, the ηI of another irrigation system can be estimated according to the abovementioned range or statistical model.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC 51579184). We are grateful to the editor and the anonymous reviewers for their valuable and constructive comments. References Arnold, J.G., Srinivasan, R., Muttiah, R.S., et al., 1998. Large area hydrologic modeling and assessment part I: model development. J. Am. Water Resour. Assoc. 34 (1), 73–89. Blanke, A., Rozelle, S., Lohmar, B., et al., 2007. Water saving technology and saving water in China. Agric. Water Manag. 87 (2), 139–150. Campadelli, P., Lanzarotti, R., Lipori, G., 2007. Efficient irrigation; inefficient communication; flawed recommendations. Irrig. Drain. 56 (4), 367–378. Chien, C., Fang, W., 2012. Modeling irrigation return flow for the return flow reuse system in paddy fields. Paddy Water Environ. 10 (3), 187–196. Cui, Y.L., Dong, B., Li, Y.H., et al., 2007. Assessment indicators and scales of water saving in agricultural irrigation. Trans. Chin. Soc. Agric. Eng. 23 (7), 1–7. Cui, Y.L., Gong, M.L., Liu, L.G., 2014. Irrigation water use efficiency index and calculation method of water-saving potential with considering reuse of return flow. J. North China Univ. Water Resour. Electr. Pow. 2 (35), 1–5. Cui, Y.L., Liu, L.G., 2015. Establish of Hydrological Model and Evaluation of Irrigation Water in Irrigation Systems. Science Press, Beijing 214pp. Cui, Y.L., Tan, F., Zheng, C.J., 2010. Analysis of irrigation efficiency and water saving potential at different scales. Adv. Water Sci. 21 (6), 788–794. Cui, Y.L., Xiong, J., 2009. Advances in assessment indicators of irrigation water use efficiency. Adv. Water Sci. 20 (4), 590–598. Guo, Y.Y., 1997. Irrigation and Drainage Engineering. China Water Power Press, Beijing 311pp. Ha, V.T.T., Quoc, V.T., Ribbe, L., 2017. Reuse potential of return flow for irrigation paddy farms in the Vu Gia Thu Bon Delta. J. Int. Sci. Publ. 5, 346–360.
4. Conclusion In this study, a new method for calculating the IE and WSP by considering the reuse of return flow was proposed. A distributed hydrological model was set up for the YSD watershed based on the modified SWAT model. After calibrating and validating the distributed hydrological model using the observed data sets, it was used to simulate the hydrological processes under various water-saving scenarios. Taking the dry year 2010 as an example, the values of ηI , IE0 , IEr , WSP0 , and WSPr were calculated under various scenarios based on the simulation results with the modified SWAT model. The results showed that: (1) an increase in the drainage area of ponds resulted in increases in ηI , IE0 , IEr , WSP0 , and WSPr ; (2) an increase in IE02 (traditional IE of water from the Zhanghe reservoir) resulted in a decrease in ηI but increases in IE0 , IEr , WSP0 , and WSPr ; (3) the water-saving irrigation modes for rice exhibited significant WSP owing to their higher ηI and irrigation 526
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Neitsch, S., Arnold, J., Kiniry, J., Williams, J., 2011. Soil and water assessment tool theoretical documentation, version 2009. Texas Water Resources Institute Technical Report No. 406. Texas A&M University System, College Station, Texas. Seckler, D., 1996. The New Era of Water Resources Management: From’ dry’ to’ Wet’ Water Savings. IWMI Research Report 1. Solomon, K.H., Davidoff, B., 1999. Relating unit and subunit irrigation performance. Trans. ASAE 42 (1), 115–122. Wang, W., Ding, Y., Shao, Q., et al., 2017. Bayesian multi-model projection of irrigation requirement and water use efficiency in three typical rice plantation region of China based on CMIP5. Agric. For. Meteorol. 232, 89–105. Wu, D., Cui, Y.L., Wang, Y.T., et al., 2019a. Reuse of return flows and its scale effect in irrigation systems based on modified SWAT model. Agric. Water Manage. 213, 280–288. Wu, D., Cui, Y.L., Xie, X.H., et al., 2019b. Improvement and testing of SWAT for multisource irrigation systems with paddy rice. J. Hydrol. 568, 1031–1041. Zulu, G., Toyota, M., Misawa, S.I., 1996. Characteristics of water reuse and its effects on paddy irrigation system water balance and the riceland ecosystem. Agric. Water Manage. 31 (3), 269–283.
Huffman, R.L., Delmar, D.F., William, J.E., et al., 2013. Soil and Water Conservation Engineering, 7th edition. ASABE., St. Joseph, Michigan. Israelsen, O.W., 1950. Irrigation Principles and Practices. John Wiley and Sons, New York 471pp. Jensen, M.E., 1977. Water conservation and irrigation systems. Proceeding of ClimateTechnology Seminar. Keller, A.A., Keller, J., 1995. Effective Efficiency: A Water Use Efficiency Concept for Allocating Freshwater Resources, Iwmi Working Papers. Li, Y.H., Dong, B., Cui, Y.L., 2005. Scale effect and the strategy for water saving irrigation. World Sci.-tech. R&D 27 (6), 31–35. Liu, L.G., Cui, Y.L., Wang, J.P., 2011. New calculation method for water-saving potential in agriculture based on water balance principle. Adv. Water Sci. 5 (22), 696–702. Liu, L.G., Cui, Y.L., Wu, X., 2013. Water use assessment indices under the influence of return flows in irrigation districts. Adv. Water Sci. 24 (4), 522–528. Mohan, S., Vijayalakshmi, D.P., 2009. Prediction of irrigation return flows through a hierarchical modeling approach. Agric. Water Manage. 96 (2), 233–246. Molden, D., 1997. Accounting for Water Use and Productivity. SWIM Paper 1. International Water Management Institute, Colombo, Sri Lanka.
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Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Irrigation efficiency and water-saving potential considering reuse of return flow
T
⁎
Di Wu, Yuanlai Cui , Yufeng Luo State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, 430072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Irrigation efficiency Water-saving potential Modified SWAT Reuse of return flow
Irrigation efficiency (IE) and water-saving potential (WSP) are two fundamental parameters for assessing water use and management in irrigation systems. A new calculation method was proposed herein to accurately estimate the IE and WSP in irrigation systems. The proposed method considers the reuse of return flow. A modified Soil and Water Assessment Tool (SWAT) was used to simulate hydrological processes under various water-saving scenarios for the Yangshudang (YSD) watershed within the Zhanghe Irrigation System (ZIS) in Hubei Province, China. The dry year of 2010 was chosen as a study case. Based on simulation results, the traditional irrigation efficiency (IE0 ) and water-saving potential (WSP0 ) as well as the irrigation efficiency taking into account the reuse of return flow (IEr ) and water-saving potential considering the reuse of return flow (WSPr ) were calculated for various scenarios. The relationships between the two IE indicators and the cause thereof, as well as the two WSP values, were analyzed and explored. The results showed that both IE and WSP were improved with the enhancement of water saving. As long as there was the reuse of return flow, IEr must be greater than IE0 . Moreover, in terms of water-saving approaches that improved the reuse rate of return flow, WSPr was determined to be greater than WSP0 , thereby suggesting that the traditional method underestimated the WSP. However, for water-saving approaches that reduced the reuse rate of return flow, WSPr was determined to be less than WSP0 , which suggested that the traditional method overestimated the WSP. The relationship between WSP0 and WSPr was attributed to the fact that WSPr was calculated by subtracting the amount of the water saved by the reuse of return flow on the basis of WSP0 , and this difference can be either positive or negative. Therefore, the managers of irrigation systems should use IEr as the actual IE but not IE0 , and use WSPr instead of WSP0 to evaluate the actual WSP.
1. Introduction Irrigation efficiency (IE) is an indicator that comprehensively reflects the efficiencies of irrigation projects, water management, and the irrigation technology at various scales. It is an important basis for correctly evaluating the effective use degree of irrigation water and the development effect of water-saving irrigation. Water-saving potential (WSP) refers to the amount of water that can be conserved in an irrigation system by adopting one or more water-saving approaches under certain socio-economic conditions. The correct calculation of WSP is of significance for guiding water-saving reforming engineering in irrigation systems. Presently, the effective use degree of irrigation water is evaluated by using the traditional IE (IE0 ). The difference between the gross irrigation water use before and after the application of water-saving approaches is used to describe the WSP, which is calculated based on the
⁎
net irrigation requirement and IE0 . This value is also known as the water-saving amount for irrigation water intake (WSP0 ) (Cui and Xiong, 2009; Cui et al., 2014). However, IE0 is calculated without considering the reuse of the return flow in irrigation systems; therefore, it cannot reflect the true effective use degree of irrigation water and the level of water management in irrigation systems. Moreover, WSP0 is not the true amount of water saved, because the amount of water leaking into the deep ground and the amount returning to the surface are also considered to be savable in the calculation of WSP0 . However, in reality, a part of these return flows still remains in the water resource system and can be reused. Therefore, this portion of return flows should not be counted as savable. Seckler (1996) indicated that water loss at the field scale might not amount to real loss because it may be reused on the larger scale. He further indicated that misconceptions regarding IE can lead to erroneous estimation of WSP. Solomon and Davidoff (1999) pointed out that the relationship between IE at the field scale and IE at
Corresponding author. E-mail address: [email protected] (Y. Cui).
https://doi.org/10.1016/j.agwat.2019.05.021 Received 1 March 2019; Received in revised form 17 May 2019; Accepted 18 May 2019 Available online 23 May 2019 0378-3774/ © 2019 Elsevier B.V. All rights reserved.
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managers. 2 To study the YSD watershed in the ZIS in Hubei Province, China, as a case. We applied the new method to calculate the IEr and WSPr of YSD under various water-saving scenarios. Additionally, we calculated IE0 and WSP0 by using traditional methods. 3 To analyze the change rules of IE0 , IEr , WSP0 and WSPr under various water-saving scenarios, and the relationships between them and the causes thereof.
the irrigation system or basin scale was not well understood, and the complex relationship between these scales was attributed to the fact that the drainage water of one farmland may be used by another farmland, i.e., there was the reuse of return flow. Li et al. (2005) and Cui et al. (2007) analyzed the different WSPs in the Zhanghe Irrigation System (ZIS) in Hubei Province, China, and determined that the WSP calculated based on water balance was much smaller than that calculated based on the IE0 and net irrigation requirement. Blanke et al. (2007) indicated that the adoption of water-saving engineering methods and technology could be effective on a relatively small scale but was extremely costly and failed to save water because a vast majority of “losses” were reused. Therefore, it is important to explore new calculation methods for IE and WSP by taking the reuse of return flow into account. Herein, IEr is the irrigation efficiency with the reuse of return flow and WSPr is the water-saving potential with reuse of the return flow. Considering these findings, many researchers have proposed new methods for calculating IE and WSP by considering the reuse of the return flow from the perspective of water resource management in irrigation systems (Jensen, 1977; Keller and Keller, 1995; Molden, 1997; Campadelli et al., 2007; Cui et al., 2010; Liu et al., 2011, 2013). Although the rationality of the associated concepts and theories have been emphasized in these methods, the application of these methods in real situations has been challenging owing to the difficulties associated with determining some of the water balance elements, especially the return flow and its reused amount. Such difficulties have hindered the application of these methods to water management in irrigation systems. Therefore, the difficulty in correctly evaluating the level of water management and water-saving potential in the irrigation systems lies in proposing practical methods to estimate IE which consider the reuse of return flow and quantifying the return flow and its reused amount. Recently, many studies have explored the process of quantifying the return flow and its reused amount (Zulu et al., 1996; Mohan and Vijayalakshmi, 2009; Chien and Fang, 2012; Ha et al., 2017). However, most of these studies only emphasized the estimation of the quantity of return flow, with limited consideration of the quantification of the reuse of the return flow. Apart from these difficulties, it is too difficult and expensive to conduct experiments for calculating the return flow and its reused amount over large-scale areas such as irrigation systems. Therefore, modeling methods are suitable for these cases that involve large-scale areas. Considering the spatial heterogeneity of irrigation systems, a distributed hydrological model is the best choice for the estimation of the return flow and its reused amount. Consequently, the development of a distributed hydrological model capable of describing hydrological processes in an irrigation system is the key to estimating the return flow and its reused amount. Wu et al. (2019a; 2019b) modified the Soil and Water Assessment Tool (SWAT) (Arnold et al., 1998; Neitsch et al., 2011) and proposed a new method for calculating the return flow and its reused amount in irrigation systems. Their method was based on the simulation results obtained from the modified SWAT model. These studies notwithstanding, there is little research on the relationship between WSPr and WSP0 . From the perspective of irrigation water management, the managers of irrigation systems hope to obtain the accurate IE and WSP. But at present, IE0 and WSP0 do not take the reuse of return flow into account, where IE0 is underestimated and WSP0 cannot represent the actual WSP, which may lead the managers to make improper water-saving measures and water management decisions. IEr and WSPr take the reuse of return flow into account and can be used to evaluate actual WSP, which can provide irrigation management decision support for the managers, while the current methods proposed by predecessors to calculate them are difficult to be used in practice. From what has been discussed above, the objectives of this study are as follows:
2. Materials and methods 2.1. Irrigation efficiency considering reuse of return flow 2.1.1. Definitions of indicators A few definitions of the irrigation efficiency from different perspectives (Israelsen, 1950; Jensen, 1977; Guo, 1997; Liu et al., 2011; Huffman et al., 2013; Cui et al., 2014) have been proposed during the past few decades. IE0 is the most widely used and is defined as follows:
IE0 =
Ws Wg
(1)
where Ws is the water applied to the soil root zone through irrigation (m3) and Wg is the gross irrigation water use, i.e., the sum of water diverted from all different water sources for irrigation (m3) (Guo, 1997; Huffman et al., 2013). As shown in Eq. (1), IE0 does not consider the impact of the reuse of return flow. Nevertheless, the return flow and its reuse are common phenomena in any irrigation system. To accurately reflect the irrigation water use efficiency, assess the level of water management, and calculate the WSP in an irrigation system, in this study, IEr is defined as follows:
IEr =
WI W + Wru = s Wg Wg
(2)
where WI is the water applied to the soil root zone through irrigation and the reuse of return flow (m3) and Wru is the water applied to the soil root zone through the reuse of return flow (m3). IEr considers the impact of the reuse of return flow not only from the water use terminal, i.e., from the soil root zone, but also from the water intake terminal, i.e., from the water sources. Therefore, IEr can also be defined as follows:
IEr =
Ws Ws = Wnew, g Wg − Wr
(3)
where Wnew, g is the gross irrigation water use that does not include the reused return flow (m3), which is equal to Wg minus the gross reused return flow in the irrigation system; Wr is the gross reused irrigation return flow (m3), which is the return flow taken from drainage channels or ponds for irrigation. In terms of the definition of IEr , as shown in Eq. (2), the numerator and denominator simultaneously contain the reused return flow, the numerator contains the net, and the denominator contains the gross. While as shown in Eq. (3), the numerator and denominator do not contain the reused return flow. In other words, the definition of IEr needs that both the numerator and denominator involve the reuse of return flow, or neither. Compared with IE0 , the calculation of IEr considers the reuse of return flow for irrigation by adding the water applied to the soil root zone through the reuse of return flow in the numerator or by subtracting the gross reused return flow in the denominator. To explore the relationship between IE0 and IEr , Eq. (2) is further represented as
WI W + Wru W W = s = s + ru Wg Wg Wg Wg Wr Wru = IE0 + ⋅ = IE0 + ηI ⋅ηr Wg Wr IEr =
1 To define and propose a practical method for calculating IEr and WSPr for providing irrigation management decision support for the 520
(4)
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Fig. 1. Study area and sub-basins division.
where ηr is the use rate of the irrigation return flow defined as the percentage of Wru in Wr ; ηI is the irrigation water reuse rate defined as the percentage of Wr in Wg . Eq. (3) can be further expanded on the basis of the equal ratios theorem in mathematics as follows:
2.2. Methods for calculating the water-saving potential In this study, the focus is on the analysis of WSP0 and WSPr . The equation for calculating WSP0 is as follows (Cui and Liu, 2015):
W W WSP0 = ⎛ n ⎞ − ⎛ n ⎞ ⎝ IE0 ⎠b ⎝ IE0 ⎠a ⎜
Ws +
Ws ⋅Wr Wg − Wr
Ws Ws Ws + IEr⋅Wr IEr = = = = Wnew, g Wg − Wr Wg − Wr + Wr Wg − Wr + Wr W IE ⋅W W = s + r r = IE0 + IEr⋅ r = IE0 + IEr⋅ηI Wg Wg Wg
IE0 1 − ηI
⎜
⎟
(7)
where Wn is the net irrigation requirement, which is generally obtained by multiplying the net irrigation quota by the irrigated area, (m3), and it is different from Ws that refers to the single use of irrigation water and excludes the reuse of return flow, whereas Wn includes the single use of irrigation water and the reuse of return flow, which corresponds to the sum of Ws and Wru ; the subscripts b and a represent before and after the application of the water-saving approaches. To calculate the WSP that accounts for the reuse of return flow, it is necessary to replace IE0 by IEr on the basis of Eq. (7):
(5)
2.1.2. Simplification of indicators As shown in Eq. (4), the calculation of IEr requires that IE0 , ηI , and ηr should be known. IE0 can be calculated by monitoring the amount of water introduced at the head of the canal and the amount of water applied to the soil root zone. Alternatively, it can be obtained from the management department of the irrigation system. ηI can be calculated by using the method proposed by Wu et al. (2019a) based on the modified SWAT model. But it is difficult to measure or estimate the value of ηr . Here it is assumed that the effective use degree of Wr is the same as that of Wg , that is to say, ηr = IEr . The equation for calculating IEr can be simplified by substituting this relationship into Eq. (4):
IEr =
⎟
W W WSPr = ⎛ n ⎞ − ⎛ n ⎞ ⎝ IEr ⎠b ⎝ IEr ⎠a ⎜
⎟
⎜
⎟
(8)
It can be seen that the main difference between the two WSPs is attributed to the difference in the methods for calculating the IE. The method used in Eq. (7) is referred to as the traditional method. The method used in Eq. (8) accounts for the reuse of the return flow. Therefore, the WSP calculated using Eq. (8) considers the change of the reused return flow after applying the water-saving approaches and can better reflect the true water-saving potential in an irrigation system.
(6)
2.3. Study area and model setup
As for Eq. (5), it can be transformed into Eq. (6) by equation transformation. This demonstrates that the final equations for calculating the irrigation efficiency are the same when considering the reuse of return flow from the two different perspectives. Furthermore, it can be seen from Eq. (6) that IEr is always greater than IE0 as long as that there is the reuse of the return flow.
2.3.1. Study area The study area was the YSD watershed in the ZIS of the Hubei Province in China. It is a relatively closed area of approximately 43.3 km2. The area is enclosed by the third main canal of the ZIS, the 521
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Table 1 Critical water depths at different growing stages of paddy rice for various rice irrigation modes. Item
Beginning date Ending date Flood irrigation Intermittent irrigation "Thin-shallow-wet-dry"
Steeping stage
May 20 May 29 20-40-80 20-40-80 10-20-80
Recovering stage
May 30 Jun 7 10-40-60 10-30-50 5-20-50
Tillering stage
Booting stage
Wet
Dry
Jun 8 Jul 5 10-40-60 10-40-60 0-25-60
Jul 6 Jul 14 20-60-80 0-0-0 0-0-0
Jul 15 Jul 30 20-60-80 20-50-70 10-35-70
Heading stage
Jul 31 Aug 9 20-60-80 20-50-70 5-35-70
Milky stage
Aug 10 Aug 18 20-60-80 10-40-60 0-25-60
Ripening stage Wet
Dry
Aug 19 Aug 26 20-60-80 0-20-30 0-10-25
Aug 27 Sep 3 0-0-0 0-0-0 0-0-0
Note: The critical water depths for irrigation modes are in the form of “Hmin − Hmax − Hp ” and the unit is millimeter.
reservoir to the fields. Therefore, two different traditional IE values were set for the different water sources. Under the present scenario, these values are IE01 = 0.90 for the local water sources and IE02 = 0.65 for the Zhanghe reservoir. The overall traditional IE is calculated as the weighted average based on the corresponding irrigation water use of the two water sources. In particular, the traditional IE associated with the Zhanghe reservoir can be increased by improving the anti-seep standards of the corresponding canals. Based on the present scenario (IE02 is 0.65), we gradually increased the traditional IE of Zhanghe reservoir by 0.05. In other words, IE02 was set to be 0.70, 0.75, 0.80, and 0.85. 3 Different rice irrigation modes. Because the study area is a typical irrigation system with paddy rice, three different irrigation modes were set for the irrigation system. These included the flood irrigation mode, intermittent irrigation mode, and the “thin-shallow-wetdry” irrigation mode. The three critical water depths of paddy fields at different growing stages for these three irrigation modes, namely, the maximum fitting depth (Hmax ), the minimum fitting depth (Hmin ), and the maximum allowable depth of impoundment after rainfall (Hp ), are shown in Table 1. 4 A comprehensive water-saving scenario. For each of the abovementioned three scenarios, there exists a specific condition where the water-saving potential is maximized. Therefore, integrating the conditions with the maximum water-saving potential associated with the three different scenarios will yield a comprehensive watersaving scenario.
first tributary canal of the third main canal, and a small branch canal, as shown in Fig. 1 (Wu et al., 2019a). The study area has a subtropical continental climate, with an annual average temperature of 17 °C, maximum temperature of 40.9 °C, and an annual mean rainfall of 821 mm. The mainland includes paddy fields, dry lands, forests, and grasslands, of which the paddy fields account for approximately 60%. The soil type is predominantly yellow-brown earth paddy soil. The study area cultivates crops such as rice, cotton, and rapeseed. Rice is the main irrigated crop and uses the intermittent irrigation mode. 2.3.2. Model setup According to Eq. (8), Wn and IEr must be determined prior to calculating WSPr . However, Eq. (6) implies that ηI must be obtained before calculating IEr . For the whole irrigation system, ηI can be calculated by using the method proposed by Wu et al. (2019a), which was based on the modified SWAT model (Wu et al., 2019b). The modified SWAT model allows for the simulation of hydrological processes under various water-saving scenarios. In this study, a distributed hydrological model was set up for the study area based on the modified SWAT model and the ηI was calculated using the method proposed by Wu et al. (2019a). The study area was divided into 10 sub-basins based on the digital elevation model (DEM) data as shown in Fig. 1. In particular, in terms of the YSD watershed, the river channels as shown in Fig. 1 are mainly used as the actual drainage channels. Furthermore, 105 hydrologic response units (HRUs) were obtained according to the type of land use, soil type distribution, and slope division. Next, a distributed hydrological model was established for the YSD watershed based on meteorological data and relevant parameters of ponds. In addition, the model was calibrated and validated using the observed discharge series in the YSD watershed outlet, the observed evapotranspiration series, and the irrigation water use from different water sources measured in the study area. For the detailed modeling, calibration, and validation process and the associated results one can refer to the study by Wu et al. (2019b).
3. Results and discussion 3.1. Results and analysis of irrigation efficiency and water-saving potential The hydrological processes under the present scenario and various water-saving scenarios described in the preceding sections were simulated with the modified SWAT model of the study area. We ranked the rainfall between 1973 and 2010 from large to small during the paddy rice growth period in the study area to calculate the rainfall frequency. We selected the dry year of 2010 (rainfall frequency is 90%) for the case study. Given that the difference between the new method proposed in this study and the traditional method is the consideration of the reuse of the return flow, ηI should be calculated under various water-saving scenarios according to the simulation results. Moreover, the corresponding values of IE0 and IEr were also calculated according to the simulation results, where IE0 was calculated as the weighted average based on irrigation water use from the local water sources and the Zhanghe reservoir, as well as the corresponding IE01 and IE02 values. IEr was calculated directly using Eq. (6). Finally, WSP0 and WSPr were calculated using Eqs. (7) and (8), respectively.
2.4. Setting up water-saving scenarios To explore the change rules of WSP0 and WSPr under various watersaving scenarios, the relationship between the two water-saving potentials, as well as the underlying causes of this relationship, four different water-saving scenarios were set up based on the characteristics of the study area: 1 Different drainage areas of ponds. The change in the drainage areas of ponds was represented by the amplification in a fraction of the sub-basin area that drains into ponds ( pnd _fr ). There is a pnd _fr for each sub-basin. Based on the present scenario (the amplification is 0%), the amplifications were set as 10%, 20%, 30%, 40%, and 50%. 2 Different traditional irrigation efficiencies. As far as the YSD watershed is concerned, the main water sources include both the local water sources (ponds and drainage channels) and the Zhanghe reservoir. The canal system is rarely used for the local water sources. However, it is required for transporting water from the Zhanghe
3.1.1. Results and analysis under various drainage areas of ponds According to the simulation results with the modified SWAT for YSD under various drainage areas of ponds, the corresponding values of ηI , IE0 , IEr , WSP0 , and WSPr were calculated as shown in Fig. 2. The following conclusions can be derived from Fig. 2: 522
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Fig. 2. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various drainage areas of ponds.
3.1.2. Results and analysis under various traditional irrigation efficiencies According to the simulation results with the modified SWAT for YSD under various IE02 values (traditional IE for water from the Zhanghe reservoir), the corresponding values of ηI , IE0 , IEr , WSP0 , and WSPr are shown in Fig. 3. The following conclusions can be derived from Fig. 3:
1 ηI increased gradually with an increase in the drainage areas of ponds because of an increase in the amount of return flow captured by the ponds with an increase in the drainage area of ponds. The increase in the return flow further resulted in an increase in the amount of the reused irrigation return flow. Consequently, increasing the drainage area of ponds could be classified as a watersaving approach that improved the reuse rate of return flow. 2 Both IE0 and IEr increased gradually with an increase in the drainage areas of ponds. This was attributed to the fact that the total amount of irrigation water required for the crops remained unchanged. Therefore, with an increase in the drainage areas of ponds, more water can be captured by the ponds. This causes an increase in the amount of irrigation water from local water sources. Correspondingly, less irrigation water was required from the Zhanghe reservoir. In addition, because the IE of the local water sources was higher than that of the Zhanghe reservoir, the overall IE was improved. IEr was determined to be greater than IE0 , that was because according to Eq. (6), as long as there was the reuse of return flow, i.e., ηI > 0, IEr was greater than IE0 . 3 Both WSP0 and WSPr increased gradually with an increase in the drainage areas of ponds. According to Eqs. (7) and (8), increases in IE0 and IEr will inevitably lead to increases in WSP0 and WSPr . In addition, WSP0 was determined to be smaller than WSPr under the water-saving scenario with an increase in the drainage areas of ponds. This indicated that the traditional method underestimated the real water-saving potential under this scenario.
1 ηI decreased gradually with an increase in IE02 . This decrease was caused by the reduction in the water conveyance losses in the canal system with an increase in IE02 , which led to a reduction in the return flow. ηI became smaller because less irrigation return flow was reused in the field. Based on this trend, an increase in IE02 could be classified as a water-saving approach that decreased the reuse rate of return flow. 2 Both IE0 and IEr increased with an increase in IE02 . Because the total amount of net irrigation requirement for the crops remained unchanged, the reduction in the water conveyance losses in the canal system from the Zhanghe reservoir to fields resulted in a smaller gross irrigation water use. This further led to a higher irrigation efficiency. In addition, Fig. 3 showed that IEr was greater than IE0 because of the reuse of return flow. 3 Both WSP0 and WSPr increased gradually with an increase in IE02 . This trend was caused by increases in IE0 and IEr as previously explained. In addition, WSP0 was determined to be greater than WSPr under the water-saving scenario with an increase in the traditional IE. This result indicated that the traditional method overestimated the real water-saving potential in this scenario.
The above results showed that increasing the drainage areas of ponds could enhance the reuse of return flow, increase the IE values and save water for irrigation. In order to increase the drainage areas of ponds in irrigation systems, the managers of irrigation systems can take some measures, such as constructing ponds and digging small ditches to divert return flow into ponds. And the managers should decide whether or not to adopt these measures in real-practice irrigation systems according to the amplification in IEr and the size of WSPr , instead of IE0 and WSP0 under the traditional method.
As mentioned above, it can be seen that an increase in IE02 could increase the overall IE values and save water for irrigation. Some measures can be taken to improve the anti-seep standards of the corresponding canals for increasing IE02 , such as lining irrigation canals, and if the investment is enough, using water pipelines instead of canals. While increasing IE02 would cut down on the reuse rate of return flow, the managers of irrigation systems do not have to increase IE02 much, because the existence of the reuse of return flow can increase IEr together with the increase of IE02 .
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Fig. 3. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various values of IE02 .
water-saving irrigation mode compared with the traditional flood irrigation mode. Therefore, water-saving potentials can be generated by replacing the traditional flood irrigation mode with water-saving irrigation modes. Moreover, ηI was determined to be higher when the intermittent irrigation mode was used compared with the “thin-shallowwet-dry” irrigation mode. However, the WSP0 and WSPr of the intermittent irrigation mode were smaller than those of the “thin-shallowwet-dry” irrigation mode. This is because replacing the flood irrigation mode with the water-saving irrigation mode not only improved the ηI value but also reduced the net irrigation water use required for the paddy rice. Furthermore, the application of the “thin-shallow-wet-dry” irrigation mode could reduce the net irrigation water requirement to a
3.1.3. Results and analysis under various irrigation modes During the analysis of the results under various irrigation modes, the flood irrigation mode was used as the basis for calculating the water-saving potentials of the two water-saving irrigation modes. Fig. 4 shows the values of ηI , IE0 , IEr , WSP0 , and WSPr under various irrigation modes. As shown in Fig. 4, the ηI values under two water-saving irrigation modes, namely, the intermittent irrigation mode and “thin-shallow-wetdry” irrigation mode, were higher than that under the flood irrigation mode. This result indicated that water-saving irrigation could be classified as a water-saving approach that increased the reuse rate of return flow. In addition, both IE0 and IEr were found to be higher under the
Fig. 4. Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under various irrigation modes. 524
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corresponding water amounts and indicators of NO. 3 sub-basin were calculated as shown in Table 3. The results showed that with the increase of drainage areas of ponds, the reused irrigation return flow increased, which made the gross irrigation water use decrease and ηI increase. In addition, the IEs and WSPs increased, and IEr was greater than IE0 and WSPr was greater than WSP0 . Combining the results and analysis on the whole YSD watershed scale, it could be found that the change rules of indicators and relationships between them in the NO. 3 sub-basin were same as that in the whole YSD watershed. That is to say, the results are correct and universally applicable for the scenarios of increasing drainage areas of ponds, and similarly, in other scenarios, the results on the sub-basin scale and whole watershed scale are consistent.
greater extent, and therefore yield a larger water-saving potential compared with the case where the intermittent irrigation mode was used. Fig. 4 showed that WSP0 was smaller than WSPr when the watersaving irrigation modes were used. In other words, the traditional method underestimated the real water-saving potential under this scenario. The above analysis indicated that the rice water-saving irrigation modes could not only improve the reuse rate of return flow and IE values, but also save water. The managers of irrigation systems can carry out the rice water-saving irrigation modes in the irrigation systems with paddy rice. Besides, some water-saving irrigation technologies like drip and sprinkle can be used for upland crops, the micro irrigation systems save water mainly by reducing the loss of water delivery system and field irrigation requirement, so the multi-source auto-irrigation module of the modified SWAT model can be used to reflect this situation, which is through increasing the traditional IE of water sources and decreasing irrigation water amount in an irrigation application, further the hydrological processes under corresponding scenarios can be simulated and the ηI , IEr and WSPr can be calculated, which can assist them to decide whether to adopt these technologies or not.
3.2. Relationship between the two water-saving potentials and cause thereof Based on the WSP0 , WSPr , and ηI values obtained under various water-saving scenarios in Section 3.1, it can be concluded that: 1 Increasing the drainage areas of ponds and using a water-saving irrigation mode for rice were the water-saving approaches that increased the reuse rate of the return flow. For these approaches, WSP0 was smaller than WSPr , suggesting that the traditional method underestimated the real water-saving potential; 2 Increasing IE02 and integrating multiple water-saving methods were water-saving approaches that reduced the reuse rate of the return flow. For these approaches, WSP0 was greater than WSPr , which suggested that the traditional method overestimated the watersaving potential.
3.1.4. Results and analysis under the comprehensive water-saving scenario As shown in Figs. 2–4, the individual water-saving scenarios with the highest water-saving potentials include: (1) the amplification of 50% in the drainage areas of ponds, (2) IE02 = 0.85, and (3) the “thinshallow-wet-dry” irrigation mode. Therefore, a comprehensive watersaving scenario can be constructed by combining these three watersaving scenarios. The values of ηI , IE0 , IEr , WSP0 , and WSPr for the comprehensive water-saving scenario were calculated based on the corresponding simulation results with the modified SWAT model and are listed in Table 2. As shown in Table 2, ηI became smaller under the comprehensive water-saving scenario compared with the present scenario. In other words, the comprehensive water-saving scenario could be classified as a water-saving approach that reduced the reuse rate of return flow. Both IE0 and IEr were determined to be higher under the comprehensive water-saving scenario compared with the values obtained under the present scenario. This indicated that there is high WSP under comprehensive water-saving scenarios compared with the present scenario. As shown in Table 2, WSP0 was greater than WSPr . This suggested that the traditional method overestimated the water-saving potential under the comprehensive water-saving scenario. Furthermore, when comparing the WSP given in Table 2 with that obtained under the individual watersaving scenarios, it was determined that combining multiple watersaving scenarios could yield a higher WSP. Therefore, combining multiple water-saving scenarios is most effective in reducing irrigation water use.
In addition, as shown in Fig. 4, the “thin-shallow-wet-dry” irrigation mode exhibited greater WSP than the intermittent irrigation mode. The WSP0 and WSPr of the “thin-shallow-wet-dry” irrigation mode calculated on the basis of the intermittent irrigation mode were 1,054,900 m3 and 872,600 m3, respectively. The ηI of the “thin-shallow-wet-dry” irrigation mode was smaller than that of the intermittent irrigation mode. This also suggested that WSP0 was greater than WSPr for the watersaving approach that reduced the reuse rate of the return flow, which was consistent with the previous conclusion. Furthermore, a similar conclusion was also drawn by Cui et al. (2014) when analyzing the relationship between the two WSPs in the Liuyuankou Irrigation System in north China. Therefore, the above conclusions might be universal. To further analyze the cause for the previously discussed relationship between WSP0 and WSPr , we substituted Eq. (6) into Eq. (8) and performed an additional analysis of the equation for calculating WSPr as follows:
W W WSPr = ⎛ n ⎞ − ⎛ n ⎞ IE r ⎝ ⎠b ⎝ IEr ⎠a W W = ⎡ n (1 − ηI ) ⎤ − ⎡ n (1 − ηI ) ⎤ ⎥ ⎥ ⎢ ⎦a ⎣ IE0 ⎦b ⎢ ⎣ IE0 ⎜
3.1.5. Results and analysis on the sub-basin scale The abovementioned results were mainly on the whole watershed scale, the results on the sub-basin scale were also needed to be analyzed. Wu et al. (2019b) verified the water balance components of NO. 3 sub-basin (Fig. 1), so it was taken as a typical sub-basin. Taking the scenarios of increasing drainage areas of ponds as examples, and according to the simulation results with modified SWAT model, the
ηI /(%)
IE0
IEr
WSP0/(10 4m3)
WSPr /(10 4m3)
Present condition Comprehensive watersaving scenario
8.02% 7.20%
0.72 0.73
0.78 0.79
/ 347.43
/ 310.41
⎜
⎟
⎛ Wn ⎞ ⎤ ⎛ Wn ⎞ ⎛ Wn ⎞ ⎤ ⎡ ⎛ Wn ⎞ =⎡ ⎢ IE0 − IE0 ⎥ − ⎢ IE0 ηI − IE0 ηI ⎥ ⎠b ⎝ ⎠a ⎦ ⎠b ⎝ ⎠a ⎦ ⎣ ⎝ ⎣⎝ ⎜
⎟
⎜
⎟
⎜
⎟
⎛ Wn ⎞ ⎛ Wn ⎞ ⎤ = WSP0 − ⎡ ⎢ IE0 ηI − IE0 ηI ⎥ ⎝ ⎠ ⎝ ⎠a ⎦ b ⎣ ⎜
Table 2 Irrigation water reuse rates, irrigation efficiencies, and the water-saving potentials under the comprehensive water-saving scenario. Water-saving scenario
⎟
⎟
⎜
⎜
⎟
⎟
(9)
As shown in Eq. (9), WSPr is calculated by subtracting the water-saving amount difference induced by the reuse of return flow on the basis of WSP0 . For water-saving approaches that maintain a constant Wn such as increasing the drainage area of ponds or increasing the traditional IE, the water-saving amount difference is a positive if ηI decreases after adopting the water-saving approaches. In this case, WSP0 is smaller than WSPr . But if ηI increases, the result will be the opposite. Moreover, for water-saving approaches that reduce Wn such as changing the irrigation 525
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Table 3 The simulation and calculation results of NO.3 sun-basin under the various drainage areas of ponds. Amplification in pnd _fr (%)
Wg /(10 4m3)
Wr /(10 4m3)
ηI /(%)
IE0
IEr
WSP0/(10 4m3)
WSPr /(10 4m3)
0 10 20 30 40 50
181.94 180.77 179.79 179.08 178.37 177.80
11.99 12.12 12.32 12.50 12.67 12.83
6.59% 6.71% 6.85% 6.98% 7.11% 7.22%
0.72 0.72 0.73 0.73 0.73 0.73
0.77 0.77 0.78 0.78 0.79 0.79
0.00 1.17 2.15 2.86 3.57 4.13
0.00 1.30 2.48 3.37 4.26 4.97
efficiency compared with the flood irrigation mode; (4) the two WSPs of a comprehensive water-saving scenario were greater than those of a single water-saving scenario. A comparison showed that IEr was greater than IE0 provided that the reused return flow was nonzero. A comparison and analysis of WSP0 and WSPr indicated that for the water-saving approaches that improved the reuse rate of the return flow, such as increasing the drainage area of ponds or using a water-saving irrigation mode for rice, the traditional method underestimated the water-saving potential. For the watersaving approaches that reduced the reuse rate of the return flow, such as increasing the traditional irrigation efficiency, the traditional method overestimated the water-saving potential. This was because the difference of water-saving amount induced by the reuse of the return flow was included in the calculation for WSPr on the basis of WSP0 , and the difference can be either positive or negative. In addition, the abovementioned change rules of the indicators and relationships between them in a sub-basin are same as that in the whole watershed, which means that the results are correct and universal. Consequently, this study systematically analyzed the relationship between IE0 and IEr as well as that between WSP0 and WSPr , for the sake of accuracy, the managers of irrigation systems should use IEr as the IE of the irrigation systems but not IE0 , and use WSPr instead of WSP0 to evaluate the actual WSP after adopting one or more water-saving approaches. Moreover, further researches are needed for better using these indicators in reality, such as simplifying the calculation of intermediate variables, etc.
mode for rice, the water-saving difference is determined by both Wn and ηI . However, the magnitude of the reduction in Wn is usually quite small to ensure that the irrigation requirements of crops can be satisfied. Consequently, the sign of the water-saving amount difference is primarily determined by ηI . According to Eq. (9) and the above description, the previously shown relationship between WSP0 and WSPr is caused by the fact that WSPr is calculated by incorporating a watersaving amount difference term induced by the reuse of return flow on the basis of WSP0 . The traditional IE can usually be obtained from the management of the irrigation systems. In this study, we proposed a new method to calculate IEr and WSPr by accounting for the reuse of the return flow. By combining the modified SWAT model proposed by Wu et al. (2019b) for multi-source irrigation systems and the calculation method for ηI based on the simulation results with the modified SWAT model (Wu et al., 2019a), we established a comprehensive set of methods for calculating IEr and WSPr .The proposed method can be used to accurately reflect the level of irrigation water management and evaluate the WSP in an irrigation system. In addition, the proposed method can be effectively used in real-practice irrigation systems. Wang et al. (2017) used Bayesian multi-model to project the water use efficiency of rice plantation region owing to the climate change, meanwhile, the modified SWAT model and the method in this study also can be used to project the IEr and WSPr under the climate change, which can provide decision support for irrigation system managers to prevent negative effects caused by climate change and guide the direction of water-saving reforming engineering. As shown in Eq. (6), if the value of ηI in an irrigation system can be obtained, IEr can be calculated based on IE0 . Although the calculation method for ηI proposed by Wu et al. (2019a) can be used in most irrigation systems, it is difficult to use in the irrigation systems where detailed data is lacking. Therefore, we can calculate the ηI of multiple irrigation systems and give the range of ηI , or further establish a statistical model between ηI and the characteristic parameters of irrigation systems, such as terrain parameters, crops planting structure, meteorological parameters and so on. In practical application, the ηI of another irrigation system can be estimated according to the abovementioned range or statistical model.
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4. Conclusion In this study, a new method for calculating the IE and WSP by considering the reuse of return flow was proposed. A distributed hydrological model was set up for the YSD watershed based on the modified SWAT model. After calibrating and validating the distributed hydrological model using the observed data sets, it was used to simulate the hydrological processes under various water-saving scenarios. Taking the dry year 2010 as an example, the values of ηI , IE0 , IEr , WSP0 , and WSPr were calculated under various scenarios based on the simulation results with the modified SWAT model. The results showed that: (1) an increase in the drainage area of ponds resulted in increases in ηI , IE0 , IEr , WSP0 , and WSPr ; (2) an increase in IE02 (traditional IE of water from the Zhanghe reservoir) resulted in a decrease in ηI but increases in IE0 , IEr , WSP0 , and WSPr ; (3) the water-saving irrigation modes for rice exhibited significant WSP owing to their higher ηI and irrigation 526
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