Contribution to irrigation from shallow water table under field conditions

agricultural water management 92 (2007) 205–210

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Contribution to irrigation from shallow water table under field conditions C. Babajimopoulos a,*, A. Panoras b, H. Georgoussis a, G. Arampatzis b, E. Hatzigiannakis b, D. Papamichail a a

Laboratory of General & Agricultural Hydraulics & Land Reclamation, Faculty of Agriculture, Aristotle University of Thessaloniki, Greece b Land Reclamation Institute, N.AG.RE.F., Greece

article info

abstract

Article history:

The mathematical model SWBACROS was applied to estimate the contribution of a shallow

Accepted 22 May 2007

groundwater to the water needs of a maize crop. The model was applied with the top and

Published on line 19 July 2007

boundary conditions defined by the observed irrigation/rainfall events and the observed water table depth. The simulated water contents of the top zone were very close to the

Keywords:

observed values. Furthermore the model was applied with an assumed free drainage bottom

Shallow water table

boundary condition. The difference of the computed water content profiles under the

Irrigation

presence and absence of the water table gave a very good estimate of the capillary rise.

Capillary rise

It was found that under the specific field conditions about 3.6 mm/day of the water in the

SWBACROS

root zone originated from the shallow water table, which amounts to about 18% of the water, which was transpired by the maize crop. # 2007 Elsevier B.V. All rights reserved.

1.

Introduction

World population today is about 6.5 billion and it is estimated that it will be increased to 9.1 billion by the year 2050 (UN, 2004). Irrigation supplies approximately 40% of the world foodstuffs on less than 18% of the arable land and has a significant future role in meeting the projected world food demand (Ayars et al., 2006). It is estimated that irrigation consumes more than 80% of the good quality water. This makes it the greatest user of water, very far from its other competitors, namely urban, industrial and environmental use. It is more than certain that competition among agricultural, urban, industrial and environmental needs will be even more intense in the near future. Any effort towards improving irrigation efficiency is worthwhile because it can lead to saving large quantities of good quality water. Shallow ground water table exists in many areas of the world. This shallow ground water can be used by plants either

by using drainage water for irrigation or through in situ use. Saline drainage ground water has been studied extensively as a supplemental source of irrigation water (Rhoades et al., 1989; Ayars et al., 1993, 2006). In situ use of ground water by crops is a more complicated matter than irrigating with drainage ground water. It depends on several factors such as depth to the water table, hydraulic properties of the soil, stage of the crop growth, ground water quality etc. Quantification of the water taken by the roots from the shallow water table is of great significance and has been a topic of extensive research in the last few decades. Wallender et al. (1979) found that 60% of the evapotranspiration (ET) of a cotton crop was extracted from a 6 dS m1 shallow water table. Ayars and Schoneman (1986) found that capillary rise of water of ECe = 10 dS m1 from a water table of 1.7–2.1 m deep contributed to up to 37% of evapotranspiration (ET) of a cotton crop.

* Corresponding author. Tel.: +30 2310 998724. E-mail address: [email protected] (C. Babajimopoulos). 0378-3774/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2007.05.009

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agricultural water management 92 (2007) 205–210

Prathapar and Qureshi (1998) observed that under shallow water table conditions irrigation can be reduced by up to 80% without affecting crop yield and increasing soil salinization. Soppe and Ayars (2003) by using weighing lysimeters maintained a saline (14 dS m1) water table at 1.5 m depth and found that ground water contributed up to 40% of daily water used by safflower crop. On a seasonal basis 25% of the total crop water use originated from the ground water. The largest contribution occurred at the end of the growing season when roots were fully developed. The applied irrigation in the presence of a water table was 46% less than irrigation applied to the crop without a water table. Kahlown et al. (2005) investigated the effect of shallow water tables on crop water requirements by using 18 large size drainage type concrete lysimeters. They found that when a water table was kept at a depth of 0.5 m, wheat met its entire water requirement from the ground water. Sunflower required only 20% of its total need from irrigation. However maize and sorghum were found to be waterlogging sensitive crops whose yield were reduced with higher water table. Most of the previous research is conducted by using weighing and drainage lysimeters. The method is very accurate but has serious limitations because of the cost of construction, operation and maintenance of the lysimeters. A common characteristic of most of this research is that it is focused on the existing conditions of the experiment and the conclusions are difficult to be extended to other situations. Thessaloniki plain, Greece, covers an area of about 100 000 hectares. The plain is cultivated with cotton (34%), maize (12%), rice (17%), sugarbeets (6%), alfalfa (3%), orchards (20%) and some other crops in smaller extent. Bad irrigation management, non functional drainage systems and also seepage from the rice fields, which are covered with water for long periods of time, result in high water table during summer in many parts of the plain. Salts accumulated in the root zone during summer are leached by rainfall in winter time when the deepening of the water table (because of the absence of rice fields) permits it. In some areas of the plain the depth to the water table during the cultivation period can be as low as 40–50 cm from the ground surface. Capillary rise is very large and contributes to crop water requirements significantly. Even though farmers reduce irrigation by taking advantage of this shallow water table, contribution of this to transpiration has never been quantified. It is obvious that if it is managed correctly, groundwater can contribute significantly to crop water needs and therefore reduce applied irrigation. The objective of this paper is to estimate ground water contribution to transpiration. Towards this goal, the mathematical model SWBACROS (Babajimopoulos et al., 1995) is used.

2.

Materials and methods

2.1.

The mathematical model

The mathematical model is based on the equation which describes the unsaturated, transient, water flow

in a crop: CðuÞ

heterogeneous

soil

under

the

presence

of

   @h @ @h ¼ KðuÞ  1  SðhÞ @t @z @z

a

(1)

where, C(u) = @u/@h is the specific moisture capacity function (1/ L), h is the pressure head (L) which is negative in unsaturated soil, K(u) is the unsaturated hydraulic conductivity (L/T), u is the volumetric soil water content (L3/L3), z is the vertical dimension directed positive downwards (L), t is the time (T) and S is the root water uptake (1/T). Eq. (1) is solved by the Douglas– Jones predictor–corrector method (Douglas and Jones, 1963; Babajimopoulos, 1991, 2000). A detailed description of the model is given by Babajimopoulos et al. (1995). It is referred here that the unsaturated hydraulic conductivity and the specific moisture capacity functions are computed as in Van Genuchten (1978, 1980). The sink term is computed as in Belmans et al. (1983) by: SðhÞ ¼ aðhÞSmax

(2)

where, a(h) is a dimensionless prescribed function of pressure head and Smax is the maximal possible water extraction by roots defined as in Feddes et al. (1978) by: Smax ¼

E Rd

(3)

where, E is the potential transpiration and Rd is the root depth. Fig. 1 is a graphical representation of Eq. (2) with the x axis representing absolute values of the pressure head. Water uptake is maximal between jh1j and jh2j and varies linearly between 0 and jh1j and between jh2j and jh3j. Actual transpiration is computed by integration of Eq. (2) over the root zone depth. The potential transpiration rate, E, is computed by: E ¼ ETP  EV

(4)

where, ETP is the potential evapotranspiration rate computed by the modified Penman method (Doorenbos and Pruitt, 1984) and EV is the potential evaporation rate from soil surface computed as in Al–Khafaf et al. (1978) by: EV ¼ ETP expð0:623LAIÞ where, LAI is the Leaf Area Index function.

Fig. 1 – General shape of the sink term S (Feddes et al., 1978).

(5)

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agricultural water management 92 (2007) 205–210

Fig. 2 – Variation of water table depth with time. Numbered vertical bars indicate each one of the six irrigation events. Fig. 3 – Soil water retention curves for 0–30 cm and 30– 60 cm depths. A maximum possible flux through the canopy is used at the top of the system (Belmans et al., 1983). Towards this computation, if irrigation/precipitation is less than 10 mm/ day then evaporation from soil surface is computed by a modified Al–Khafaf et al. (1978) equation: EVA ¼ EV t0:6  EVðt  1Þ0:6

(6)

where, t is the time in days after the dry period starts. The model has been validated several times in the past against data from the literature (Babajimopoulos, 1991, 2000) and field data (Babajimopoulos et al., 1995, 2000). It has been shown that it describes unsaturated flow very realistically.

2.2.

Area of study and field data

The model was applied in a six acres experimental field situated in the area of the Thessaloniki plain (w: 408 390 1300 , l: 228 460 0100 WGS84 projection) and cultivated with maize. The crop was planted on 14 April 2004 and the experimental field was irrigated by a set of furrows. The electrical conductivity (EC) of the shallow water table was 0.93 dS/m. The water table was observed at an average depth of 0.58 m below soil surface and its variations were recorded by two piezometers. Fig. 2 shows the variation of water table depth with time. Irrigation and precipitation events are also shown in the same figure. The soil retention curve was estimated by pressure cell and pressure plate methods (Childs, 1969). The values of us and ur along with the coefficients a and n of the Van Genuchten model were determined by non linear regression of the soil water retention data. The results of the mechanical composition and the values of the parameters describing the hydraulic properties of the soil are given in Table 1. The soil

retention curves of the 0–30 and 30–60 cm depths are shown in Fig. 3. The h1 value of the function S(h) was calculated assuming a 5% air filled space in the 0–30 cm layer as proposed by Feddes et al. (1978). According to this assumption h1 = 1.0 m. No data were available for the values of the parameters h2 and h3 of S(h). Therefore these values were obtained from Feddes et al. (1978) and are the following: h2 = 6.0 m, h3 = 150.0 m. Although these values were derived from experiments conducted under quite different conditions than the ones in Thessaloniki plain, it was found that under our field data their variation produces marginal differences to the computed transpiration. Root depth was measured by isolating the root zone and carefully washing it. The Leaf Area Index (LAI) function with respect to calendar day was measured by a Delta-T Devices SunScan Canopy Analysis System. A logistic equation of the form (Dale et al., 1980): A¼

Amax 1 þ aexp½bðt  to Þ

(7)

was used to calculate the variation of LAI and rooting depth (RD) with calendar day t. In this equation t0 is the calendar day when calculations start, a and b are regression parameters and Amax is the maximum value of the variable named A. The parameters appearing in Eq. (7) are defined in Table 2. Rooting depth and LAI functions with time are graphically presented in Fig. 4. Climatic data were obtained from an adjacent weather station. In addition to rainfall, a total of 73.9 mm of water were applied by the farmer during six irrigation events to cover crop water needs. Disturbed soil samples were used to measure gravimetrically the soil water content of the 0–30 and 30–60 cm

Table 1 – Mechanical composition and hydraulic parameters of the soil Depth (cm) 0–30 30–60

Sand (%)

Silt (%)

Clay (%)

USDA texture

us (cm3 cm3)

Van Genuchten retention parameters ur (cm3 cm3)

a (m1)

33.2 32.4

47.0 53.5

19.8 14.1

L SiL

0.452 0.517

0.000 0.160

2.497 0.349

n 1.089 1.736

Ks (m/day) 0.5 0.003

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agricultural water management 92 (2007) 205–210

Table 2 – Values of parameters of Eq. (7) for the determination of Leaf Area Index (LAI) and rooting depth (RD) A (LAI or RD) 2

LAI (m m2)

Amax a b t0

111  t  219

219 < t  255

4.7 14971.6 0.18076 111

4.7 0.242  103 0.2752 219

RD (cm) 105  t  255 50 223331.2 0.156892 105

layers twice a week. Undisturbed soil samples were used to determine the saturated hydraulic conductivity of each layer using a falling head permeameter.

3.

Fig. 5 – Observed water content (Qobs), simulated water content under the presence and absence of a water table and groundwater contribution to crop needs. Numbered vertical bars indicate each one of the six irrigation events.

Results

Groundwater contribution to irrigation was estimated by the following way: the model was applied with the top and bottom boundary conditions defined by the observed irrigation/rainfall events and water table depths, respectively. The simulated water contents were very close to the observed values. Furthermore a free drainage bottom boundary condition was assumed. The difference of the computed water content profiles under the presence and the absence of the water table was an estimation of the water used by the crop because of the capillary rise. The simulation period started on 15 June 2004 and ended on 11 September 2004. The simulation ended on that particular day because after that an extreme rainfall event (82.8 mm) flooded the experimental field for seven consecutive days. Since there is a time lag between a rainfall or irrigation event and its effect on water table depth, it was decided that the groundwater contribution be estimated for a period starting a few days after the day of the initial condition of the simulation. Thus although the simulation period lasted from 15 June to 11 September, the period over which the groundwater contribution was evaluated was from 1 July to 11 September. Fig. 5 shows the water content of the top 30 cm layer of the soil predicted by the model and the observed water content. The average mean error was 0.11  103 cm3/cm3 and the coefficient of variation (CV) was 6.7%. In the same figure, the predicted water content under an assumed free drainage bottom boundary condition is also shown.

Fig. 4 – LAI and rooting depth functions with time.

The difference between the two simulated curves of this figure is due to the capillary rise because of the presence of the water table (Fig. 2). Groundwater contribution to the water content of the top zone for the simulated period is also shown in Fig. 5. For the period from 1 July to 11 September 2004, this contribution varies between 1.68 and 4.83 mm day1 with an average value of 3.6 mm day1. This sums up to about 283.8 mm. During the simulation period the applied irrigation was 73.9 mm and precipitation was 22.6 mm. This precipitation fell in 11 different days in very small amounts. Therefore, groundwater contribution and irrigation give a sum of 357.7 mm. It is pointed out that potential evapotranspiration for the period of 1 July to 11 September 2004 as computed by the modified Penman method is 344 mm. Rooting depth below surface and groundwater contribution are shown in Fig. 6. Groundwater contribution is higher when roots are fully developed. It is worth mentioning in this figure that a large amount of the water that originated either from irrigation or from ground water was added to the top zone when the crop was not fully developed. Therefore this amount is not contributing significantly to the transpiration of the crop. That is obvious in Fig. 7 where the computed transpiration under the presence and the absence of a water table is shown. It appears that about 18% of the transpired water is due to the shallow water table. That seems to be in contrast with the fact that the water of the root zone for the simulated period was 357.7 mm while the estimated evapotranspiration was 344 mm. Actually it is not because a large amount out of the 357.7 mm was added to the top zone before the time the crop was able to use it. It is apparent that groundwater contribution, as it is estimated in this work, is restricted to the specific field conditions. Even though our experimental field represents a typical soil type in the plain of Thessaloniki, application of the model to the rest of the soil types of the plain will lead to a very good estimate of the groundwater contribution under different field conditions. This will certainly lead to a much better management of irrigation in the plain of Thessaloniki.

agricultural water management 92 (2007) 205–210

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Fig. 6 – Variation of rooting depth and groundwater contribution with time. Numbered vertical bars indicate each one of the six irrigation events.

Fig. 7 – Cumulative potential and actual transpiration under the presence and absence of a water table. Numbered vertical bars indicate each one of the six irrigation events.

4.

Conclusions

Crop water use from shallow water tables is affected by several factors such as depth to water table, rooting depth, ground water quality, crop salt tolerance, soil type, irrigation frequency and application depth. Because of this complexity it is impossible to conduct experiments that cover all the factors at once and the research is focused on only a single component, i.e. water use relative to the water table depth, or ground water quality or soil type (Ayars et al., 2006). The method which was used in this paper is relatively easy, does not have the limitations of the weighing lysimeters (cost of construction, operation, maintenance) and can study several factors at the same time e.g. depth to the water table, crop growth stage, soil type. It was found that under the specific field conditions groundwater contribution to the root zone was about 3.6 mm/day. This amounts to about 18% of the transpired water for the period 1 July to 11 September 2004. Therefore, groundwater is a very significant source of water to cover crop demands. The contribution of groundwater increases as root length increases. This directs us to the conclusion that cultivation practice should lead to more frequent irrigation events with small amounts of water during the period when root length is small. The interval between irrigation events can be increased when roots have been fully developed taking advantage of the presence of the groundwater.

The model has been applied in a six acres field in the plain of Thessaloniki. Therefore, the results of this work are limited at most to the soil type at which this field belongs. It is intended that the model be applied to the prevailing soil types in the plain of Thessaloniki. This will lead to a very good estimate of the groundwater contribution to crop needs and therefore to a significant improvement of the irrigation efficiency.

references

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