- Email: [email protected]
Conjunctive use of reclaimed water and groundwater in crop rotations
Agricultural Water Management 116 (2013) 228–234
Contents lists available at SciVerse ScienceDirect
Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Conjunctive use of reclaimed water and groundwater in crop rotations Saif A. Al Khamisi a,∗ , S.A. Prathapar b , M. Ahmed c a
Ministry of Agriculture and Fisheries, PO Box 50, PC 121 Seeb, Oman International Water Management Institute, New Delhi Office, NASC Complex, DPS Marg, Pusa, New Delhi 110012, India c Department of Soils, Water and Agricultural Engineering, PO Box 34, PC 123, Al-Khoud, Sultan Qaboos University, Oman b
a r t i c l e
i n f o
Article history: Received 17 January 2012 Accepted 28 July 2012 Available online 14 August 2012 Keywords: Conjunctive water use Reclaimed water Groundwater Arid Salinity Crop rotation
a b s t r a c t Irrigated agriculture in Oman relies solely on groundwater and Aflaj (Falaj is a canal system, which provides water for a community of farmers for domestic and agricultural use). With the increasing scarcity of freshwater available to agriculture, the need to use of reclaimed water (RW) from Sewage Treatment Plants (STP) in agriculture has increased. In this study, we explored how RW from an STP can be used directly, without Aquifer Storage and Recovery, as a source of irrigation water in conjunction with groundwater for agriculture. Average data from Muscat, Oman in the years from 1996 to 2010 was used for calculation of crop water requirement. Wheat, cowpea and maize were chosen as crops to be grown in rotation through the year. Using RW irrigation conjunctively with groundwater cropping areas of wheat, cowpea and maize can be increased by 323, 250 and 318% respectively, against utilization RW only. Of the total irrigation requirement 57.6% was met with reclaimed water (RW) and 42.4% was met with groundwater (GW). Therefore, it is recommended that decision makers should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, rather than piping it to areas where groundwater is saline and unsuitable for irrigation. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Irrigated agriculture is practiced on 5.8% of Oman’s land area (The World Bank, 2008), and it relies solely on groundwater. The net annual recharge to groundwater is estimated at 1276 Mm3 (million cubic meter), whereas the consumption is about 1645 Mm3 with a shortage of 378 Mm3 (MRMWR, 2009). The soils are coarse textured (sandy or coarse loamy) with a high infiltration rate. The soil pH is moderately too strong alkaline and the organic matter is very low. The cultivated area was 58,850 ha in 2004, of which 12,793 ha consisted of annual crops and 46,057 ha of permanent crops (Frenken, 2005). Oman counts five distinct agricultural regions. Going roughly from north to south, they include the Musandam Peninsula, the Batinah coast, the valleys and the high plateau of the eastern region, the interior oases, and the Dhofar region. Over half of the agricultural area is located on AlBatinah Plain in the north covering about 4% of the area of the country. With the increasing scarcity of freshwater available to agriculture in Oman, the need to use reclaimed water (RW) in agriculture has increased. It is a major challenge to optimize the benefits of reclaimed water as a source of both water and the nutrients it contains. Effects of RW irrigation on soil properties (Castro
∗ Corresponding author. Tel.: +968 2414 3151; fax: +968 26893097. E-mail address: [email protected] (S.A. Al Khamisi). 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.07.013
et al., 2011), groundwater (Sophocleous et al., 2009), and crop productivity (Qadir et al., 2007) have been reported extensively in literature. Reclaimed wastewater irrigation offers an opportunity to be applied in arid and semi-arid regions (Xu et al., 2010) to satisfy both water shortage and deficiency of nutrients of soils. In many parts of the world, Aquifer Storage and Recovery (ASR) of RW have been practiced. The Hueco Bolson aquifer in Texas receives a very limited natural recharge. A recharge project started full operation in 1985 and treated up to 7500 m3 d−1 of RW to drinking water standards for injection into the aquifer (El Paso Water Utilities, 2011). The Whittier Narrows Water Reclamation Plant provides treatment for 170,000 m3 of RW per day, making up 35% of the total recharge to the groundwater basin (SDLAC (Sanitation Districts of Los Angeles County), 2011). Orange County Water District as part of its groundwater management, uses approximately 21 Mm3 RW for year-round recharge (Orange Country Water District, 2011). In the West basin municipal water district of California, reclaimed water is injected in coastal aquifers for prevention of seawater intrusion (West Basin, 2011). Perth, the capital of Western Australia, currently discharges more than 100 Mm3 of RW to sea every year. Pilot scale infiltration galleries with RW are being trailed (CSIRO Land and Water 2011). On the other hand, ASR has some disadvantages. In most cases, only a part – approximately 65% – of the recharged water is recovered. This will depend on aquifer characteristics, residence time of recharged water, and distance to extraction wells from ASR
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
locations. Quality of recharging water could lead to changes in physical and chemical characteristics of the soil and aquifer. Some impurities, such as microbes, heavy metals or trace elements, if present in recharging water will contaminate the aquifer, and will be very expensive to contain and clean. To avoid these problems, the injected water must undergo treatment processes that can satisfy stringent quality standards. Currently most of the Gulf countries, including Oman use tertiary RW to irrigate public gardens and green strips in urban area. Irrigation demands of these amenities vary during the year, while the supply of reclaimed water from sewage treatment plants (STP) remains reasonably steady. The surplus reclaimed water can be used to grow seasonal crops, or stored for future in aquifer, or disposed in the sea. Often, water quality requirements for aquifer storage are very stringent, and meeting these requirements is costly. Prathapar et al. (2009) showed that, if mandated, an advanced treatment of the excess effluent before ASR is technically feasible in Muscat, Oman. However, a financial analysis shows that the project will not be profitable because of very high cost of treatment process. Disposing RW to the sea is not prudent either. Therefore there is a need to maximize the use of RW, by growing short season crops throughout the year and supplement RW with groundwater. By doing so, RW will not be disposed to the sea, it need not be injected to the aquifer, stress on groundwater will be minimal, while crop production will be maximized. Oman Wastewater Services Company (OWSC) has been given the right to build, operate and maintain a world class wastewater system in Muscat, Oman. In 2009, it produced 38,270 m3 d−1 (14 Mm3 y−1 ) of RW. Currently, OWSC is not authorized to store this water in aquifer, and surface storages are also infeasible due to soil and climatic conditions in Oman. However, OWSC supplies RW to Muscat Municipality to irrigate amenities such as nature strips and public parks. Currently, RW in surplus is discharged to the sea. In order to better utilize the RW, the OWSC is planning to pipe it to AlBatinah region of Oman. In this study, we explore how RW from an STP can be used directly (without ASR) as a source of irrigation water in conjunction with groundwater for agriculture in AlBatinah Region. Oman, a country without surface water resources such as rivers and irrigation canals, relies on groundwater for irrigations. Unfortunately, groundwater and flat lands for irrigation are found only along the coastal areas, and indiscriminate use of groundwater has resulted in extensive seawater intrusion. Approximately 52% of the flat lands, which were irrigated during the seventies and eighties are no longer suitable for irrigation because of secondary soil salinity, and the groundwater underlined is hyper-saline. AlBatinah region extends 270 km along the Gulf of Oman from Muscat to UAE border, north of Shinas. This area is of vital importance to the agricultural economy of the Sultanate of Oman. Rainfall on the mountains is the major source of fresh water that recharges aquifers under AlBatinah area. With the introduction of modern drilling and groundwater pumping systems, the agriculture has expanded on both sides of AlBatinah Coastal Highway. Increased pumping has lowered groundwater levels and the quality of water has deteriorated due to salt-water intrusion. Subsequently approximately 52% of the region is abandoned from irrigated agriculture due to soil and groundwater salinity (Al-Barwani and Helmi, 2006). Under these circumstances, RW could be used to reclaim saline soils and grow crops. If such practice is adopted, RW will remain the sole source of irrigation water, because the groundwater is very saline (EC > 20 dS m−1 ). On the other hand piping this RW from STPs to unsalinized lands where the groundwater is also suitable for irrigation, groundwater quality may complement RW and meet variation in seasonal evaporative demand. Conjunctive water use may be defined as the two-way shift between surface and groundwater resources to meet irrigation demand, based on variation in evaporative demand and supply
229
of surface water. It is assumed that available groundwater will be pumped to meet any deficit in the supply of surface water. In traditional canal irrigation systems, supply of surface water will depend on irrigation system characteristics, such as the extent of storage and regulation, travel time from storage to point of demand, and capacity of irrigation canals. Anyone of these aspects may alter the supply of irrigation water, and therefore the supply is often unsteady. The evaporative demand on the other hand will vary due to climatic and crop characteristics. In other words, in most canal irrigated systems, demand and supply will vary continuously. However, if the source of surface water is RW from an STP, then the supply will remain steady, while the demand will vary with climate and crop characteristics. Therefore, management strategies must respond to a steady supply of RW, varying demand for irrigation water, minimize groundwater pumping, and minimize the need for ASR or disposal of RW to the sea. There had been very limited studies which reported conjunctive use of RW and groundwater. Ejaz and Peralta (1995) developed a simulation/optimization model to determine the use of RW in conjunction with river and groundwater while ensuring water quality constraints are met. Conjunctive use of groundwater, river water, and treated municipal water has been promoted in the Shepparton Irrigation Region (Surapaneni and Olsson, 2002). The situation in Muscat poses several interesting questions. How can a constant year round supply of RW from an STP be used in agriculture, without discharged to the sea or recharged to the aquifer, while crop water requirement is met? What type of crop rotation and cropping area will maximize the use of RW? How much groundwater should be conjunctively used with available RW? Should the RW be used to reclaim areas with saline soils and saline groundwater, or be used in areas where groundwater and soils are not saline? This study aims to answer these questions using data available for Muscat Region of Oman. In this study we did not consider the economic cost and benefit aspect, the environmental impacts in terms of soil and groundwater quality degradation, and the possibility of yield reduction (100% yield for all crops was assumed and leaching fraction calculated accordingly). We also made effort to calculate leaching fraction on a daily basis taking into consideration the mixing of treated wastewater and slightly saline groundwater. We assumed constant irrigation water salinity of various levels. 2. Theory The crop water requirement (CWR) is defined as the depth (or amount) of water needed to meet the water loss through evapotranspiration. The crop water requirement mainly depends on the climate, the crop type, and days after planting (DAP), reflecting the growth stage of the crop. Suppose that crop water requirement of a crop looks like the curve shown in Fig. 1a. Since the supply of RW is constant, except for the day (or period) when the supply of RW and maximum crop water requirement is equal, there will be a surplus (S) of RW. If we minimize the surplus RW (Fig. 1b), by increasing the cropped area, then water from another source (e.g. groundwater) need to meet the deficit (D). If we wish to fully utilize RW in irrigation, then except for the day (or period) when the supply of RW and minimum crop water requirement are equal, water from another source need to be used to supplement RW (Fig. 1c). 3. Materials and methods 3.1. Estimating daily crop water requirement Reclaimed water produced in 2009 (38,268 m3 day−1 ) by Oman Wastewater Service Company (OWSC) was selected as input to the
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
CWR (m3 d-1)
230
(a)
(b)
ARW
D
(c) D ARW
ARW
S
DAP
DAP
DAP
Fig. 1. Potential scenarios of conjunctive water management using RW and groundwater.
analysis. Average climatic data of 15 years (1996–2010) for Muscat were obtained from Masters (2011). They were input to calculate daily reference evapotranspiration (ET0 ), using Penman–Monteith equation (FAO, 1998). For maximum utilization of reclaimed water (RW), three crops that cover seasons around the year (January–December) and adapted to Muscat climate were selected. Selected crops were wheat (Triticum aestivum L.), cowpea (Vignaun guiculata L.) and maize (Zea mays L.). Their daily crop evapotranspiration demand (ETc ) was calculated by multiplying ET0 by corresponding crop factors (Kc ) (FAO, 1998) presented in Table 1. In Oman, traditionally, wheat is planted in winter and followed by cowpea or sorghum in summer. Wheat needs cool winters and hot summers, bright sunny days with lower humidity. These conditions prevail in Oman from mid-November till mid-March to April. The optimum temperature required is about 25 ◦ C and the maximum about 30–32 ◦ C. Prior to 1970s, approximately 1700 ha in Oman were under wheat, but it has declined to approximately 500 ha in 2005. During this period, gross wheat production in Oman has declined from 3000 metric tons to less than 1000 metric tons. Interestingly, during the same period, the average wheat yield has increased from 1.2 t ha−1 to 3.2 t ha−1 (FAOStat, 2011). Decline in wheat area is possibly attributable to the increase in oil revenues, importation of wheat or flour and the increase in yield per ha is attributable to increased use of fertilizers, high yielding varieties and irrigation. Since, the average yield per ha is comparable to the world average, it is reasonable to consider that wheat can be successfully grown in Oman. Furthermore, following a surge in wheat prices and short supplies in international markets, the Government of Oman is encouraging wheat production locally. In northern Oman, cowpea is sown during the middle of March. For green forage it can be cut 2–3 times in a season. The green forage yield on an average is 40–50 tons ha−1 . Maize is one of the most important annual forage grass cultivated in Sultanate of Oman during late summer along with Sorghum. Cowpea and maize are used for both green fodder and grains. Local varieties are
predominantly grown in Oman interior and Western Hajar. The period from planting and the immature cobs is about 80 days and the forage is relished by the animals. In some cases, the green forage is made into silage. The maize forage yield is 40–50 tons ha−1 (Akhtar and Nadaf, 2002).
3.2. Estimating leaching requirement for RW and groundwater irrigation The salinity of reclaimed water from Muscat STPs was reported to be between 0.97 and 1.03 dS m−1 . The reclaimed water is targeting AlBatinah region where the groundwater is highly saline in some areas. However, most agriculture is located in good groundwater zones (<1.0 dS m−1 ). Application of irrigation water means application of some amount of salt. These salts have to be washed out of the rootzone by water percolating to the sub-soil. Leaching is generally provided by an excess of irrigation water in the field. A common way to express total salinity is in term of electrical conductivity (EC). EC of soil samples is usually determined in saturation extract (ECse ). In this study, four levels of irrigation water salinity were assumed, i.e. ECw of 0.5, 1.0, 1.5 and 2.0 dS m−1 . The average leaching requirement fractions were calculated using Eq. (1) and the irrigation requirement was calculated using Eq. (2). The equation was obtained from the leaching requirement computed as recommended by Ayers and Westcot (1985). Letey and Feng (2007) concluded that the LR as determined from traditional steady-state approaches is higher than necessary. Recent research has shown that LR calculated that way seems to give higher values than what is actually needed in the field (Corwin et al., 2007).
LR =
ECi 5ECse − ECi
(1)
Table 1 Crop factors (Kc ) of selected crops at different growth stages (FAO, 1998). Crops
Stage Init. (Lini )
Dev. (Ldev ) 20
No. of days in each stage Planting Harvesting Kc
30
Wheat
20
Cowpea
No. of days in each stage Planting Harvesting Kc
20
Maize
No. of days in each stage Planting Harvesting Kc
Mid (Lmid )
Late (Llate )
40
30
Mid November Mid-March 0.7
0.7
1.15
35
35
0.4 30
Mid-March 31st of July 1.05
1.05
1.05
30
50/30
0.6 10
1st of August Mid November 1.15
1.15
1.2
0.9
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
IR =
ETc 1 − LR
(2)
where IR is the irrigation requirement (mm), ETc the crop evaporative requirement (mm) and LR is the leaching requirement for 100% yield.
ETo
14 Evapotranspiration mm day-1
where LR is the average leaching fraction, ECi the electrical conductivity of irrigation water (dS m−1 ) and ECse is the average root zone salinity at saturation paste extract (dS m−1 ). The leaching fraction at which soil salinity ECse equals the threshold salinity for any crop is corresponding to the leaching requirement (LR) for 100% yield of that crop. The average soil salinity ECse (threshold salinity) for wheat crop is 6.0 dS m−1 . However, it is 2.0 dS m−1 and 1.7 dS m−1 for cowpea and maize respectively (DeHayr et al., 1997).
231
ETc
12 Wheat
10
Cowpea
Maize
Wheat
8 6 4 2 0
Fig. 2. Estimated average ET0 and ETc (mm d−1 ) from 1996 to 2010 for Muscat climatic conditions and selected crops.
The area of each crop that can be grown with RW alone (Minimum Area) was estimated by dividing the volume of RW by IRmax for the crop during its season (Eq. (3)). Similarly the maximum area of crop that can be grown by using all available RW, and supplemented by groundwater was estimated by dividing the volume of RW by IRmin for the crop during the season (Eq. (4)). Total RW to be used and surplus RW during the season can be estimated using Eqs. (5)–(7). Minimum Area =
RW produced IRmax
(3)
Maximum Area =
RW produced IRmin
(4)
SIW =
n
IR × Area
(if (RWi − IWi > 0)
(6)
(if (IWi − RWi > 0)
(7)
i=1
SGW =
n
IWi − RWi
0.5 dS/m 1.0 dS/m 1.5 dS/m 2.0 dS/m
(5)
n
RWi − IWi
20 18 16 14 12 10 8 6 4 2 0
Fig. 3. Estimated average ETc including leaching requirement for the selected crops under four irrigation water salinity conditions.
i=1
SRW =
ETc mm day-1
3.3. Estimating areas and demand for RW and groundwater
i=1
where Minimum Area is the area of a crop that can be grown with RW alone (m2 ), Maximum Area is the area of a crop that can be grown without any excess of RW (m2 ), IRmax the maximum daily crop requirement as depth (m d−1 ), IRmin the minimum daily crop requirement as depth (m d−1 ), RW the reclaimed Water (m3 d−1 ), IW the water used for irrigation (m3 d−1 ), SRW the surplus RW (m3 season−1 ), SIW the water used for irrigation (m3 season−1 ), SGW the groundwater requirement (m3 season−1 ) and n is the duration of growing season (d).
with maximum of 9.63, 9.96, 10.52 and 11.33 mm day−1 for 0.5, 1.0, 1.5 and 2.0 dS m−1 respectively. The minimum irrigation requirements were 1.60, 1.66, 1.75 and 1.89 mm day−1 for 0.5, 1.0, 1.5 and 2.0 dS m−1 respectively. Leaching fraction for the selected crops under four irrigation water salinities was estimated using Eq. (1) (Table 2). The threshold, percent yield reduction per unit increase above soil salinity (ECe ), was obtained from Tanji and Kielen (2002). Minimum, maximum areas, RW and groundwater to be used by each crop estimated using Eqs. (4)–(8) are presented in Table 3. Variation in minimum areas reflects variation among crop types and seasons in which they are grown (Fig. 4). Wheat is generally grown in cooler season, and its Kc values are lower than the other two crops (Table 1). Variations of area (ha) of each crop with the increase of the assumed irrigation water salinity (ECw ) are presented in Table 4. The reduction in total area was 8, 19.5 and 33%
Daily average reference evapotranspiration (ET0 ) and crop evapotranspiration (ETc ) for the rotation of crops are presented in Fig. 2. Average annual ET0 for 15 years (1996–2010) was 2033 mm, with a maximum of 8.68 mm d−1 on 15th of May, and a minimum of 3.22 mm d−1 on 26th of December. Estimated cumulative ETc for selected crops was 1896 mm, with a maximum of 9.46 mm on 14th of May and a minimum of 1.58 mm on 14th February. In general ETc followed the same pattern as ET0 , except between crops, when ET0 was higher than ETc . Average ETc including leaching requirement for the selected crops of the assumed four irrigation water salinity conditions are presented in Fig. 3. Estimated cumulative irrigation requirements (ETc ) were 1929, 1997, 2108 and 2269 mm,
Volume Mm3 day-1
4. Results and discussion
40000
R W Irrigated
35000
R W Excess
30000 25000 20000 15000 10000 5000 0
Fig. 4. RW used for irrigation and excess to be disposed during the selected crops growing period.
232
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
Table 2 Leaching fraction of the selected crops under four irrigation water salinities. Crop
Wheat Cowpea Maize
Crop threshold (ECse )
6.0 2.0 1.7
Irrigation water salinity (ECw ) 0.50 dS m−1
1.0 dS m−1
1.5 dS m−1
2.0 dS m−1
0.017 0.053 0.063
0.034 0.111 0.133
0.053 0.176 0.214
0.071 0.250 0.308
Table 3 Ranges of areas (ha) of each crop, reclaimed water use (RWU) and groundwater (GWU) use, and potential excess SRW. Scenario
Variable
Cowpea
Maize
Minimum
Area (ha) RWU (Mm3 ) SRW (Mm3 )
694.63 2.62 2.02
312.57 3.24 1.43
346.21 3.53 1.14
Maximum
Area (ha) RWU (Mm3 ) GWU (Mm3 ) SRW (Mm3 )
2245.31 4.63 3.82 0.00
781.92 4.67 3.44 0.00
753.83 4.67 3.01 0.00
Wheat
when the irrigation water salinity increased from 0.5 to 1.0, 1.5 and 2.0 dS m−1 level respectively at the minimum scenario. However, at the maximum scenario, it was 5, 12 and 21% when the irrigation water salinity increased from 0.5 to 1.0, 1.5 and 2.0 dS m−1 level respectively Fig. 4 presents volumes of RW used for irrigation, and excess to be disposed to the sea if cropping is carried out in minimum areas. In the event that a decision was made to pipe the RW to saline groundwater zones, these volumes would be superfluous and will be discharged to the sea. As expected on three days, one per crop, all RW were used to meet crop evaporative requirement, and in all days except these three, considerable volumes of RW were to be discharged to the sea. Irrigation volumes ranged from 11,839 to 38,268 m3 day−1 , and volumes to be discharged to the sea ranged from 0 to 26,429 m3 day−1 . Cumulative RW used for irrigation and cumulative RW to be discharged to the sea 9.38 and 4.59 Mm3 , respectively. In other words, almost 33% of the reclaimed water should be discharged to the sea. Fig. 5 presents volumes of RW used for irrigation, and groundwater to be pumped daily if cropping is carried out in maximum areas. As expected on three days, one per crop, all RW were used to meet crop evaporative requirement. In all days except the three, considerable volumes of GW need to be pumped to meet crop water requirements. Cumulative RW and groundwater used for irrigation were 13.97 and 10.27 Mm3 , respectively. Irrigation volumes ranged from 38,267 to 123,695 m3 day−1 ; groundwater to be pumped ranged from 0 to 85,428 m3 day−1 . Results from the analysis above show that by using RW conjunctively with groundwater cropping area can be increased (against RW use only) from 694.63 ha to 2245.31 ha (323% increase) of wheat, 313 ha to 782 ha (250% increase) of cowpea and 346 ha to 754 ha (318% increase) of maize. Of the total irrigation
120000
RW Irrigated GW Irrigated
Volume (m3 day-1)
100000 80000 60000 40000 20000 0
Fig. 5. Reclaimed water and groundwater used for irrigation of the selected crops.
requirement 24.24 Mm3 , 57.6% was met with RW and 42.4% was met with groundwater. Therefore, the decision makers should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, than piping it to areas where groundwater is saline and unsuitable for irrigation. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones. Suppose the area of each crop is increased at a same rate from their respective minimum areas to maximum areas, changes to crop water requirement, RW and GW used in irrigation with increase in cropping areas are presented in Table 5. Fig. 6 shows that the percentage increases in GW to meet crop water requirement, with the increase of cropping area. It also shows that the percentage reduction in RW excess as cropping area is increased. This type of information may be useful, in case groundwater availability is limited. For example, if the cropping area is
Table 4 Variation of area (ha) of each crop with the increase of irrigation water salinity (ECw ). Scenario
Irrigation water salinity ECw (dS m−1 )
Minimum
0.5 1.0 1.5 2.0
795.81 768.75 728.01 676.32
397.77 353.62 291.38 218.53
470.31 407.76 320.50 221.79
1663.89 1530.13 1339.89 1116.64
Maximum
0.5 1.0 1.5 2.0
2344.21 2264.51 2144.49 1992.23
885.72 787.41 648.82 486.62
1285.39 1241.69 1175.88 1092.39
4515.33 4293.61 3969.19 3571.24
Selected crops Wheat
Cowpea
Maize
Total
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
233
Table 5 Potential changes to reclaimed water (RW) and groundwater (GW) use with increase in cropping areas. Total area (ha)
Wheat area (ha)
Cowpea area (ha)
Maize area (ha)
CWR (Mm3 )
RW irrigated (Mm3 )
RW excess (Mm3 )
GW irrigated (Mm3 )
1200 1480 1760 2040 2320 2600 2880 3160 3440 3720 4000
600 780 960 1140 1320 1500 1680 1860 2040 2220 2400
300 350 400 450 500 550 600 650 700 750 800
300 350 400 450 500 550 600 650 700 750 800
8.425 10.130 11.835 13.541 15.246 16.951 18.657 20.362 22.067 23.773 25.478
8.425 10.083 11.330 12.106 12.720 13.218 13.549 13.793 13.921 13.965 13.968
5.543 3.884 2.638 1.862 1.248 0.750 0.419 0.175 0.047 0.003 0.000
0.000 0.047 0.505 1.435 2.526 3.733 5.108 6.569 8.146 9.808 11.510
50%
%RW Excess
45% 40% 35% 30% 25% 20% 15% 10% 5% 0% 0
500
1000
1500
2000
2500
3000
3500
4000
4500
increase) of wheat, 313–782 ha (250% increase) of cowpea and 346–754 ha (318% increase) of maize. Of the total irrigation requirement 24.24 Mm3 , 57.6% was met with RW and 42.4% was met with groundwater. Therefore, the planners should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, than piping it to areas where groundwater is saline and unsuitable for irrigation. They could plan on developing a maximum of developing 2245 ha (maximum area for wheat) to be irrigated conjunctively with RW and groundwater. During warmer seasons, a part of it may be used to grow cowpea or maize. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones.
Total area cropped (ha) Fig. 6. RW% excess and CWR% met with groundwater for increase in cropped area (ha).
2320 ha (1320 ha of wheat, 500 ha of cowpea and 500 ha of maize), then 83.4% of the CWR will be met with RW and only 16.6% of the CWR will be required from groundwater (GW). 5. Summary With the increasing scarcity of freshwater available to agriculture, the need to use reclaimed water (RW) in agriculture has increased. Currently most of the Gulf States, including Oman use RW to irrigate public gardens and green strips in urban area. Irrigation demands of these amenities vary during the year, while the supply of reclaimed water from sewage treatment plants (STP) remains reasonably steady. The surplus RW can be used to grow seasonal crops, or stored for future in aquifer, or disposed in the sea. Often, water quality requirements for aquifer storage are very stringent, and meeting these requirements is costly. Therefore there is a need to maximize the use of RW, by growing short season crops throughout the year, changing the area under cultivation of such crops, and supplement RW with groundwater. By doing so, RW will not be disposed to the sea, it need not be injected to the aquifer, stress on groundwater will be minimal, while crop production will be maximized. In order to better utilize the RW, the Oman Wastewater Services Company (OWSC) is planning to pipe it to the AlBatinah region of Oman. The area is of vital importance to the agricultural economy of the Sultanate of Oman, but 52% of the area is abandoned due to soil and groundwater salinity. In this study, how RW from an STP can be used directly (without ASR) as a source of irrigation water in conjunction with groundwater for agriculture in AlBatinah Region was explored. Average weather data (1996–2010) from Muscat, Oman, was used. Wheat, cowpea and maize were chosen as crops to be grown in rotation through the year. Results show that by using RW conjunctively with groundwater cropping area can be increased from 695 ha to 2245 ha (323%
References Akhtar, M., Nadaf, S.K., 2002. Consolidated Results of the Experiments in Field Crops (1971–2000). Directorate General of Agriculture, Ministry of Agriculture and Fisheries, Sultanate of Oman. Al-Barwani, A., Helmi, T., 2006. Sea water intrusion in a coastal aquifer: a case study for the area between Seeb and Suwaiq, Sultanate of Oman. SQU Journal of Agricultural and Marine Sciences 11, 55–69. Ayers, R.S., Westcot, D.W., 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29. Food and Agriculture Organization of the United Nations, Rome, 174 pp. Castro, E., Manas, M.P., Heras, J., 2011. Effects of wastewater irrigation on soil properties and turfgrass growth. Water Science and Technology 63, 1678– 1688. Corwin, D.L., Rhoades, J.D., Simunek, J., 2007. Leaching requirement for soil salinity control: steady-state versus transient models. Agricultural Water Management 90 (3), 165–180. CSIRO Land and Water Home. Web. Water Use and Reuse Stream Projects. (06.07.11). DeHayr, R., Diatloff, N., Gordon, I., 1997. Produced by: Resource Sciences Centre DNRQ97089. The State of Queensland. ISSN 1327-5364 (02.07.12). Ejaz, M.S., Peralta, R.C., 1995. Maximizing conjunctive use of surface and ground water under surface water quality constraints. Advanced Water Resources 18, 67–75. El Paso Water Utilities, 2011. “Reclaimed Water.” – Public Service Board. Web. (06.07.11). FAO, 1998. Allen, R.G., Pereira, L.S., Raes, D., Smith, M. Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements – Irrigation and Drainage Paper 56. FAO, Rome. FAOStat., Food and Agriculture Organization of the United Nations. (06.07.11). Frenken, K. (Ed.). Estimates from FAO, 2005. Irrigation in the Middle East Region in Figures. AQUASTAT Survey—2008. Letey, J., Feng, G.L., 2007. Dynamic versus steady-state approaches to evaluate irrigation management of saline waters. Agricultural Water Management 91 (1–3), 1–10. Masters, J., Welcome to Weather Underground: Weather Underground. Web. (06.07.11). MRMWR, 2009. Water Resources in Oman. Ministry of Regional Municipalities and Water Resources, Sultanate of Oman, http://www.mrmwr.gov.om/. Orange County Water District. Web. Groundwater Recharge. (06.07.11). Prathapar, S.A., Ahmed, M., Abdalla, O., Zekri, S., Al Jabri, S., Al Maktoomi, A., AlShuely, M., 2009. Managed Aquifer Recharge of Treated Waste Water in Oman. Report Submitted to Oman Waste Water Company. 69 pp.
234
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
Qadir, M., Sharma, B.R., Bruggeman, A., Choukr-Alla, R., Karajeh, F., 2007. Nonconventional water resources and opportunities for water augmentation to achieve food security in water scarce countries. Agricultural Water Management 87, 2–22. SDLAC (Sanitation Districts of Los Angeles County), 2011. Wastewater Facilities. SDLAC Website – Homepage. Web.
Tanji, K.K., Kielen, N.C., 2002. FAO Irrigation and Drainage Paper 61: Agricultural Drainage Water Management in Arid and Semi-Arid Areas. FAO, Rome. ISSN 0254-5264. West Basin Home. Web. Water Reliability 2020. (06.07.11). World Bank. Web. The World Bank Data Oman 2008. (12.11.11). Xu, J., Wu, L., Chang, A.C., Zhang, Y., 2010. Impact of long-term reclaimed wastewater irrigation on agricultural soils: a preliminary assessment. Journal of Hazardous Materials 183 (2010), 780–786.
Contents lists available at SciVerse ScienceDirect
Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Conjunctive use of reclaimed water and groundwater in crop rotations Saif A. Al Khamisi a,∗ , S.A. Prathapar b , M. Ahmed c a
Ministry of Agriculture and Fisheries, PO Box 50, PC 121 Seeb, Oman International Water Management Institute, New Delhi Office, NASC Complex, DPS Marg, Pusa, New Delhi 110012, India c Department of Soils, Water and Agricultural Engineering, PO Box 34, PC 123, Al-Khoud, Sultan Qaboos University, Oman b
a r t i c l e
i n f o
Article history: Received 17 January 2012 Accepted 28 July 2012 Available online 14 August 2012 Keywords: Conjunctive water use Reclaimed water Groundwater Arid Salinity Crop rotation
a b s t r a c t Irrigated agriculture in Oman relies solely on groundwater and Aflaj (Falaj is a canal system, which provides water for a community of farmers for domestic and agricultural use). With the increasing scarcity of freshwater available to agriculture, the need to use of reclaimed water (RW) from Sewage Treatment Plants (STP) in agriculture has increased. In this study, we explored how RW from an STP can be used directly, without Aquifer Storage and Recovery, as a source of irrigation water in conjunction with groundwater for agriculture. Average data from Muscat, Oman in the years from 1996 to 2010 was used for calculation of crop water requirement. Wheat, cowpea and maize were chosen as crops to be grown in rotation through the year. Using RW irrigation conjunctively with groundwater cropping areas of wheat, cowpea and maize can be increased by 323, 250 and 318% respectively, against utilization RW only. Of the total irrigation requirement 57.6% was met with reclaimed water (RW) and 42.4% was met with groundwater (GW). Therefore, it is recommended that decision makers should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, rather than piping it to areas where groundwater is saline and unsuitable for irrigation. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Irrigated agriculture is practiced on 5.8% of Oman’s land area (The World Bank, 2008), and it relies solely on groundwater. The net annual recharge to groundwater is estimated at 1276 Mm3 (million cubic meter), whereas the consumption is about 1645 Mm3 with a shortage of 378 Mm3 (MRMWR, 2009). The soils are coarse textured (sandy or coarse loamy) with a high infiltration rate. The soil pH is moderately too strong alkaline and the organic matter is very low. The cultivated area was 58,850 ha in 2004, of which 12,793 ha consisted of annual crops and 46,057 ha of permanent crops (Frenken, 2005). Oman counts five distinct agricultural regions. Going roughly from north to south, they include the Musandam Peninsula, the Batinah coast, the valleys and the high plateau of the eastern region, the interior oases, and the Dhofar region. Over half of the agricultural area is located on AlBatinah Plain in the north covering about 4% of the area of the country. With the increasing scarcity of freshwater available to agriculture in Oman, the need to use reclaimed water (RW) in agriculture has increased. It is a major challenge to optimize the benefits of reclaimed water as a source of both water and the nutrients it contains. Effects of RW irrigation on soil properties (Castro
∗ Corresponding author. Tel.: +968 2414 3151; fax: +968 26893097. E-mail address: [email protected] (S.A. Al Khamisi). 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.07.013
et al., 2011), groundwater (Sophocleous et al., 2009), and crop productivity (Qadir et al., 2007) have been reported extensively in literature. Reclaimed wastewater irrigation offers an opportunity to be applied in arid and semi-arid regions (Xu et al., 2010) to satisfy both water shortage and deficiency of nutrients of soils. In many parts of the world, Aquifer Storage and Recovery (ASR) of RW have been practiced. The Hueco Bolson aquifer in Texas receives a very limited natural recharge. A recharge project started full operation in 1985 and treated up to 7500 m3 d−1 of RW to drinking water standards for injection into the aquifer (El Paso Water Utilities, 2011). The Whittier Narrows Water Reclamation Plant provides treatment for 170,000 m3 of RW per day, making up 35% of the total recharge to the groundwater basin (SDLAC (Sanitation Districts of Los Angeles County), 2011). Orange County Water District as part of its groundwater management, uses approximately 21 Mm3 RW for year-round recharge (Orange Country Water District, 2011). In the West basin municipal water district of California, reclaimed water is injected in coastal aquifers for prevention of seawater intrusion (West Basin, 2011). Perth, the capital of Western Australia, currently discharges more than 100 Mm3 of RW to sea every year. Pilot scale infiltration galleries with RW are being trailed (CSIRO Land and Water 2011). On the other hand, ASR has some disadvantages. In most cases, only a part – approximately 65% – of the recharged water is recovered. This will depend on aquifer characteristics, residence time of recharged water, and distance to extraction wells from ASR
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
locations. Quality of recharging water could lead to changes in physical and chemical characteristics of the soil and aquifer. Some impurities, such as microbes, heavy metals or trace elements, if present in recharging water will contaminate the aquifer, and will be very expensive to contain and clean. To avoid these problems, the injected water must undergo treatment processes that can satisfy stringent quality standards. Currently most of the Gulf countries, including Oman use tertiary RW to irrigate public gardens and green strips in urban area. Irrigation demands of these amenities vary during the year, while the supply of reclaimed water from sewage treatment plants (STP) remains reasonably steady. The surplus reclaimed water can be used to grow seasonal crops, or stored for future in aquifer, or disposed in the sea. Often, water quality requirements for aquifer storage are very stringent, and meeting these requirements is costly. Prathapar et al. (2009) showed that, if mandated, an advanced treatment of the excess effluent before ASR is technically feasible in Muscat, Oman. However, a financial analysis shows that the project will not be profitable because of very high cost of treatment process. Disposing RW to the sea is not prudent either. Therefore there is a need to maximize the use of RW, by growing short season crops throughout the year and supplement RW with groundwater. By doing so, RW will not be disposed to the sea, it need not be injected to the aquifer, stress on groundwater will be minimal, while crop production will be maximized. Oman Wastewater Services Company (OWSC) has been given the right to build, operate and maintain a world class wastewater system in Muscat, Oman. In 2009, it produced 38,270 m3 d−1 (14 Mm3 y−1 ) of RW. Currently, OWSC is not authorized to store this water in aquifer, and surface storages are also infeasible due to soil and climatic conditions in Oman. However, OWSC supplies RW to Muscat Municipality to irrigate amenities such as nature strips and public parks. Currently, RW in surplus is discharged to the sea. In order to better utilize the RW, the OWSC is planning to pipe it to AlBatinah region of Oman. In this study, we explore how RW from an STP can be used directly (without ASR) as a source of irrigation water in conjunction with groundwater for agriculture in AlBatinah Region. Oman, a country without surface water resources such as rivers and irrigation canals, relies on groundwater for irrigations. Unfortunately, groundwater and flat lands for irrigation are found only along the coastal areas, and indiscriminate use of groundwater has resulted in extensive seawater intrusion. Approximately 52% of the flat lands, which were irrigated during the seventies and eighties are no longer suitable for irrigation because of secondary soil salinity, and the groundwater underlined is hyper-saline. AlBatinah region extends 270 km along the Gulf of Oman from Muscat to UAE border, north of Shinas. This area is of vital importance to the agricultural economy of the Sultanate of Oman. Rainfall on the mountains is the major source of fresh water that recharges aquifers under AlBatinah area. With the introduction of modern drilling and groundwater pumping systems, the agriculture has expanded on both sides of AlBatinah Coastal Highway. Increased pumping has lowered groundwater levels and the quality of water has deteriorated due to salt-water intrusion. Subsequently approximately 52% of the region is abandoned from irrigated agriculture due to soil and groundwater salinity (Al-Barwani and Helmi, 2006). Under these circumstances, RW could be used to reclaim saline soils and grow crops. If such practice is adopted, RW will remain the sole source of irrigation water, because the groundwater is very saline (EC > 20 dS m−1 ). On the other hand piping this RW from STPs to unsalinized lands where the groundwater is also suitable for irrigation, groundwater quality may complement RW and meet variation in seasonal evaporative demand. Conjunctive water use may be defined as the two-way shift between surface and groundwater resources to meet irrigation demand, based on variation in evaporative demand and supply
229
of surface water. It is assumed that available groundwater will be pumped to meet any deficit in the supply of surface water. In traditional canal irrigation systems, supply of surface water will depend on irrigation system characteristics, such as the extent of storage and regulation, travel time from storage to point of demand, and capacity of irrigation canals. Anyone of these aspects may alter the supply of irrigation water, and therefore the supply is often unsteady. The evaporative demand on the other hand will vary due to climatic and crop characteristics. In other words, in most canal irrigated systems, demand and supply will vary continuously. However, if the source of surface water is RW from an STP, then the supply will remain steady, while the demand will vary with climate and crop characteristics. Therefore, management strategies must respond to a steady supply of RW, varying demand for irrigation water, minimize groundwater pumping, and minimize the need for ASR or disposal of RW to the sea. There had been very limited studies which reported conjunctive use of RW and groundwater. Ejaz and Peralta (1995) developed a simulation/optimization model to determine the use of RW in conjunction with river and groundwater while ensuring water quality constraints are met. Conjunctive use of groundwater, river water, and treated municipal water has been promoted in the Shepparton Irrigation Region (Surapaneni and Olsson, 2002). The situation in Muscat poses several interesting questions. How can a constant year round supply of RW from an STP be used in agriculture, without discharged to the sea or recharged to the aquifer, while crop water requirement is met? What type of crop rotation and cropping area will maximize the use of RW? How much groundwater should be conjunctively used with available RW? Should the RW be used to reclaim areas with saline soils and saline groundwater, or be used in areas where groundwater and soils are not saline? This study aims to answer these questions using data available for Muscat Region of Oman. In this study we did not consider the economic cost and benefit aspect, the environmental impacts in terms of soil and groundwater quality degradation, and the possibility of yield reduction (100% yield for all crops was assumed and leaching fraction calculated accordingly). We also made effort to calculate leaching fraction on a daily basis taking into consideration the mixing of treated wastewater and slightly saline groundwater. We assumed constant irrigation water salinity of various levels. 2. Theory The crop water requirement (CWR) is defined as the depth (or amount) of water needed to meet the water loss through evapotranspiration. The crop water requirement mainly depends on the climate, the crop type, and days after planting (DAP), reflecting the growth stage of the crop. Suppose that crop water requirement of a crop looks like the curve shown in Fig. 1a. Since the supply of RW is constant, except for the day (or period) when the supply of RW and maximum crop water requirement is equal, there will be a surplus (S) of RW. If we minimize the surplus RW (Fig. 1b), by increasing the cropped area, then water from another source (e.g. groundwater) need to meet the deficit (D). If we wish to fully utilize RW in irrigation, then except for the day (or period) when the supply of RW and minimum crop water requirement are equal, water from another source need to be used to supplement RW (Fig. 1c). 3. Materials and methods 3.1. Estimating daily crop water requirement Reclaimed water produced in 2009 (38,268 m3 day−1 ) by Oman Wastewater Service Company (OWSC) was selected as input to the
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
CWR (m3 d-1)
230
(a)
(b)
ARW
D
(c) D ARW
ARW
S
DAP
DAP
DAP
Fig. 1. Potential scenarios of conjunctive water management using RW and groundwater.
analysis. Average climatic data of 15 years (1996–2010) for Muscat were obtained from Masters (2011). They were input to calculate daily reference evapotranspiration (ET0 ), using Penman–Monteith equation (FAO, 1998). For maximum utilization of reclaimed water (RW), three crops that cover seasons around the year (January–December) and adapted to Muscat climate were selected. Selected crops were wheat (Triticum aestivum L.), cowpea (Vignaun guiculata L.) and maize (Zea mays L.). Their daily crop evapotranspiration demand (ETc ) was calculated by multiplying ET0 by corresponding crop factors (Kc ) (FAO, 1998) presented in Table 1. In Oman, traditionally, wheat is planted in winter and followed by cowpea or sorghum in summer. Wheat needs cool winters and hot summers, bright sunny days with lower humidity. These conditions prevail in Oman from mid-November till mid-March to April. The optimum temperature required is about 25 ◦ C and the maximum about 30–32 ◦ C. Prior to 1970s, approximately 1700 ha in Oman were under wheat, but it has declined to approximately 500 ha in 2005. During this period, gross wheat production in Oman has declined from 3000 metric tons to less than 1000 metric tons. Interestingly, during the same period, the average wheat yield has increased from 1.2 t ha−1 to 3.2 t ha−1 (FAOStat, 2011). Decline in wheat area is possibly attributable to the increase in oil revenues, importation of wheat or flour and the increase in yield per ha is attributable to increased use of fertilizers, high yielding varieties and irrigation. Since, the average yield per ha is comparable to the world average, it is reasonable to consider that wheat can be successfully grown in Oman. Furthermore, following a surge in wheat prices and short supplies in international markets, the Government of Oman is encouraging wheat production locally. In northern Oman, cowpea is sown during the middle of March. For green forage it can be cut 2–3 times in a season. The green forage yield on an average is 40–50 tons ha−1 . Maize is one of the most important annual forage grass cultivated in Sultanate of Oman during late summer along with Sorghum. Cowpea and maize are used for both green fodder and grains. Local varieties are
predominantly grown in Oman interior and Western Hajar. The period from planting and the immature cobs is about 80 days and the forage is relished by the animals. In some cases, the green forage is made into silage. The maize forage yield is 40–50 tons ha−1 (Akhtar and Nadaf, 2002).
3.2. Estimating leaching requirement for RW and groundwater irrigation The salinity of reclaimed water from Muscat STPs was reported to be between 0.97 and 1.03 dS m−1 . The reclaimed water is targeting AlBatinah region where the groundwater is highly saline in some areas. However, most agriculture is located in good groundwater zones (<1.0 dS m−1 ). Application of irrigation water means application of some amount of salt. These salts have to be washed out of the rootzone by water percolating to the sub-soil. Leaching is generally provided by an excess of irrigation water in the field. A common way to express total salinity is in term of electrical conductivity (EC). EC of soil samples is usually determined in saturation extract (ECse ). In this study, four levels of irrigation water salinity were assumed, i.e. ECw of 0.5, 1.0, 1.5 and 2.0 dS m−1 . The average leaching requirement fractions were calculated using Eq. (1) and the irrigation requirement was calculated using Eq. (2). The equation was obtained from the leaching requirement computed as recommended by Ayers and Westcot (1985). Letey and Feng (2007) concluded that the LR as determined from traditional steady-state approaches is higher than necessary. Recent research has shown that LR calculated that way seems to give higher values than what is actually needed in the field (Corwin et al., 2007).
LR =
ECi 5ECse − ECi
(1)
Table 1 Crop factors (Kc ) of selected crops at different growth stages (FAO, 1998). Crops
Stage Init. (Lini )
Dev. (Ldev ) 20
No. of days in each stage Planting Harvesting Kc
30
Wheat
20
Cowpea
No. of days in each stage Planting Harvesting Kc
20
Maize
No. of days in each stage Planting Harvesting Kc
Mid (Lmid )
Late (Llate )
40
30
Mid November Mid-March 0.7
0.7
1.15
35
35
0.4 30
Mid-March 31st of July 1.05
1.05
1.05
30
50/30
0.6 10
1st of August Mid November 1.15
1.15
1.2
0.9
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
IR =
ETc 1 − LR
(2)
where IR is the irrigation requirement (mm), ETc the crop evaporative requirement (mm) and LR is the leaching requirement for 100% yield.
ETo
14 Evapotranspiration mm day-1
where LR is the average leaching fraction, ECi the electrical conductivity of irrigation water (dS m−1 ) and ECse is the average root zone salinity at saturation paste extract (dS m−1 ). The leaching fraction at which soil salinity ECse equals the threshold salinity for any crop is corresponding to the leaching requirement (LR) for 100% yield of that crop. The average soil salinity ECse (threshold salinity) for wheat crop is 6.0 dS m−1 . However, it is 2.0 dS m−1 and 1.7 dS m−1 for cowpea and maize respectively (DeHayr et al., 1997).
231
ETc
12 Wheat
10
Cowpea
Maize
Wheat
8 6 4 2 0
Fig. 2. Estimated average ET0 and ETc (mm d−1 ) from 1996 to 2010 for Muscat climatic conditions and selected crops.
The area of each crop that can be grown with RW alone (Minimum Area) was estimated by dividing the volume of RW by IRmax for the crop during its season (Eq. (3)). Similarly the maximum area of crop that can be grown by using all available RW, and supplemented by groundwater was estimated by dividing the volume of RW by IRmin for the crop during the season (Eq. (4)). Total RW to be used and surplus RW during the season can be estimated using Eqs. (5)–(7). Minimum Area =
RW produced IRmax
(3)
Maximum Area =
RW produced IRmin
(4)
SIW =
n
IR × Area
(if (RWi − IWi > 0)
(6)
(if (IWi − RWi > 0)
(7)
i=1
SGW =
n
IWi − RWi
0.5 dS/m 1.0 dS/m 1.5 dS/m 2.0 dS/m
(5)
n
RWi − IWi
20 18 16 14 12 10 8 6 4 2 0
Fig. 3. Estimated average ETc including leaching requirement for the selected crops under four irrigation water salinity conditions.
i=1
SRW =
ETc mm day-1
3.3. Estimating areas and demand for RW and groundwater
i=1
where Minimum Area is the area of a crop that can be grown with RW alone (m2 ), Maximum Area is the area of a crop that can be grown without any excess of RW (m2 ), IRmax the maximum daily crop requirement as depth (m d−1 ), IRmin the minimum daily crop requirement as depth (m d−1 ), RW the reclaimed Water (m3 d−1 ), IW the water used for irrigation (m3 d−1 ), SRW the surplus RW (m3 season−1 ), SIW the water used for irrigation (m3 season−1 ), SGW the groundwater requirement (m3 season−1 ) and n is the duration of growing season (d).
with maximum of 9.63, 9.96, 10.52 and 11.33 mm day−1 for 0.5, 1.0, 1.5 and 2.0 dS m−1 respectively. The minimum irrigation requirements were 1.60, 1.66, 1.75 and 1.89 mm day−1 for 0.5, 1.0, 1.5 and 2.0 dS m−1 respectively. Leaching fraction for the selected crops under four irrigation water salinities was estimated using Eq. (1) (Table 2). The threshold, percent yield reduction per unit increase above soil salinity (ECe ), was obtained from Tanji and Kielen (2002). Minimum, maximum areas, RW and groundwater to be used by each crop estimated using Eqs. (4)–(8) are presented in Table 3. Variation in minimum areas reflects variation among crop types and seasons in which they are grown (Fig. 4). Wheat is generally grown in cooler season, and its Kc values are lower than the other two crops (Table 1). Variations of area (ha) of each crop with the increase of the assumed irrigation water salinity (ECw ) are presented in Table 4. The reduction in total area was 8, 19.5 and 33%
Daily average reference evapotranspiration (ET0 ) and crop evapotranspiration (ETc ) for the rotation of crops are presented in Fig. 2. Average annual ET0 for 15 years (1996–2010) was 2033 mm, with a maximum of 8.68 mm d−1 on 15th of May, and a minimum of 3.22 mm d−1 on 26th of December. Estimated cumulative ETc for selected crops was 1896 mm, with a maximum of 9.46 mm on 14th of May and a minimum of 1.58 mm on 14th February. In general ETc followed the same pattern as ET0 , except between crops, when ET0 was higher than ETc . Average ETc including leaching requirement for the selected crops of the assumed four irrigation water salinity conditions are presented in Fig. 3. Estimated cumulative irrigation requirements (ETc ) were 1929, 1997, 2108 and 2269 mm,
Volume Mm3 day-1
4. Results and discussion
40000
R W Irrigated
35000
R W Excess
30000 25000 20000 15000 10000 5000 0
Fig. 4. RW used for irrigation and excess to be disposed during the selected crops growing period.
232
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
Table 2 Leaching fraction of the selected crops under four irrigation water salinities. Crop
Wheat Cowpea Maize
Crop threshold (ECse )
6.0 2.0 1.7
Irrigation water salinity (ECw ) 0.50 dS m−1
1.0 dS m−1
1.5 dS m−1
2.0 dS m−1
0.017 0.053 0.063
0.034 0.111 0.133
0.053 0.176 0.214
0.071 0.250 0.308
Table 3 Ranges of areas (ha) of each crop, reclaimed water use (RWU) and groundwater (GWU) use, and potential excess SRW. Scenario
Variable
Cowpea
Maize
Minimum
Area (ha) RWU (Mm3 ) SRW (Mm3 )
694.63 2.62 2.02
312.57 3.24 1.43
346.21 3.53 1.14
Maximum
Area (ha) RWU (Mm3 ) GWU (Mm3 ) SRW (Mm3 )
2245.31 4.63 3.82 0.00
781.92 4.67 3.44 0.00
753.83 4.67 3.01 0.00
Wheat
when the irrigation water salinity increased from 0.5 to 1.0, 1.5 and 2.0 dS m−1 level respectively at the minimum scenario. However, at the maximum scenario, it was 5, 12 and 21% when the irrigation water salinity increased from 0.5 to 1.0, 1.5 and 2.0 dS m−1 level respectively Fig. 4 presents volumes of RW used for irrigation, and excess to be disposed to the sea if cropping is carried out in minimum areas. In the event that a decision was made to pipe the RW to saline groundwater zones, these volumes would be superfluous and will be discharged to the sea. As expected on three days, one per crop, all RW were used to meet crop evaporative requirement, and in all days except these three, considerable volumes of RW were to be discharged to the sea. Irrigation volumes ranged from 11,839 to 38,268 m3 day−1 , and volumes to be discharged to the sea ranged from 0 to 26,429 m3 day−1 . Cumulative RW used for irrigation and cumulative RW to be discharged to the sea 9.38 and 4.59 Mm3 , respectively. In other words, almost 33% of the reclaimed water should be discharged to the sea. Fig. 5 presents volumes of RW used for irrigation, and groundwater to be pumped daily if cropping is carried out in maximum areas. As expected on three days, one per crop, all RW were used to meet crop evaporative requirement. In all days except the three, considerable volumes of GW need to be pumped to meet crop water requirements. Cumulative RW and groundwater used for irrigation were 13.97 and 10.27 Mm3 , respectively. Irrigation volumes ranged from 38,267 to 123,695 m3 day−1 ; groundwater to be pumped ranged from 0 to 85,428 m3 day−1 . Results from the analysis above show that by using RW conjunctively with groundwater cropping area can be increased (against RW use only) from 694.63 ha to 2245.31 ha (323% increase) of wheat, 313 ha to 782 ha (250% increase) of cowpea and 346 ha to 754 ha (318% increase) of maize. Of the total irrigation
120000
RW Irrigated GW Irrigated
Volume (m3 day-1)
100000 80000 60000 40000 20000 0
Fig. 5. Reclaimed water and groundwater used for irrigation of the selected crops.
requirement 24.24 Mm3 , 57.6% was met with RW and 42.4% was met with groundwater. Therefore, the decision makers should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, than piping it to areas where groundwater is saline and unsuitable for irrigation. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones. Suppose the area of each crop is increased at a same rate from their respective minimum areas to maximum areas, changes to crop water requirement, RW and GW used in irrigation with increase in cropping areas are presented in Table 5. Fig. 6 shows that the percentage increases in GW to meet crop water requirement, with the increase of cropping area. It also shows that the percentage reduction in RW excess as cropping area is increased. This type of information may be useful, in case groundwater availability is limited. For example, if the cropping area is
Table 4 Variation of area (ha) of each crop with the increase of irrigation water salinity (ECw ). Scenario
Irrigation water salinity ECw (dS m−1 )
Minimum
0.5 1.0 1.5 2.0
795.81 768.75 728.01 676.32
397.77 353.62 291.38 218.53
470.31 407.76 320.50 221.79
1663.89 1530.13 1339.89 1116.64
Maximum
0.5 1.0 1.5 2.0
2344.21 2264.51 2144.49 1992.23
885.72 787.41 648.82 486.62
1285.39 1241.69 1175.88 1092.39
4515.33 4293.61 3969.19 3571.24
Selected crops Wheat
Cowpea
Maize
Total
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
233
Table 5 Potential changes to reclaimed water (RW) and groundwater (GW) use with increase in cropping areas. Total area (ha)
Wheat area (ha)
Cowpea area (ha)
Maize area (ha)
CWR (Mm3 )
RW irrigated (Mm3 )
RW excess (Mm3 )
GW irrigated (Mm3 )
1200 1480 1760 2040 2320 2600 2880 3160 3440 3720 4000
600 780 960 1140 1320 1500 1680 1860 2040 2220 2400
300 350 400 450 500 550 600 650 700 750 800
300 350 400 450 500 550 600 650 700 750 800
8.425 10.130 11.835 13.541 15.246 16.951 18.657 20.362 22.067 23.773 25.478
8.425 10.083 11.330 12.106 12.720 13.218 13.549 13.793 13.921 13.965 13.968
5.543 3.884 2.638 1.862 1.248 0.750 0.419 0.175 0.047 0.003 0.000
0.000 0.047 0.505 1.435 2.526 3.733 5.108 6.569 8.146 9.808 11.510
50%
%RW Excess
45% 40% 35% 30% 25% 20% 15% 10% 5% 0% 0
500
1000
1500
2000
2500
3000
3500
4000
4500
increase) of wheat, 313–782 ha (250% increase) of cowpea and 346–754 ha (318% increase) of maize. Of the total irrigation requirement 24.24 Mm3 , 57.6% was met with RW and 42.4% was met with groundwater. Therefore, the planners should consider piping RW to areas where groundwater of good quality is available to conjunctively use and meet crop water requirements, than piping it to areas where groundwater is saline and unsuitable for irrigation. They could plan on developing a maximum of developing 2245 ha (maximum area for wheat) to be irrigated conjunctively with RW and groundwater. During warmer seasons, a part of it may be used to grow cowpea or maize. This will prevent disposal of RW to the sea and minimize stress on fresh groundwater zones.
Total area cropped (ha) Fig. 6. RW% excess and CWR% met with groundwater for increase in cropped area (ha).
2320 ha (1320 ha of wheat, 500 ha of cowpea and 500 ha of maize), then 83.4% of the CWR will be met with RW and only 16.6% of the CWR will be required from groundwater (GW). 5. Summary With the increasing scarcity of freshwater available to agriculture, the need to use reclaimed water (RW) in agriculture has increased. Currently most of the Gulf States, including Oman use RW to irrigate public gardens and green strips in urban area. Irrigation demands of these amenities vary during the year, while the supply of reclaimed water from sewage treatment plants (STP) remains reasonably steady. The surplus RW can be used to grow seasonal crops, or stored for future in aquifer, or disposed in the sea. Often, water quality requirements for aquifer storage are very stringent, and meeting these requirements is costly. Therefore there is a need to maximize the use of RW, by growing short season crops throughout the year, changing the area under cultivation of such crops, and supplement RW with groundwater. By doing so, RW will not be disposed to the sea, it need not be injected to the aquifer, stress on groundwater will be minimal, while crop production will be maximized. In order to better utilize the RW, the Oman Wastewater Services Company (OWSC) is planning to pipe it to the AlBatinah region of Oman. The area is of vital importance to the agricultural economy of the Sultanate of Oman, but 52% of the area is abandoned due to soil and groundwater salinity. In this study, how RW from an STP can be used directly (without ASR) as a source of irrigation water in conjunction with groundwater for agriculture in AlBatinah Region was explored. Average weather data (1996–2010) from Muscat, Oman, was used. Wheat, cowpea and maize were chosen as crops to be grown in rotation through the year. Results show that by using RW conjunctively with groundwater cropping area can be increased from 695 ha to 2245 ha (323%
References Akhtar, M., Nadaf, S.K., 2002. Consolidated Results of the Experiments in Field Crops (1971–2000). Directorate General of Agriculture, Ministry of Agriculture and Fisheries, Sultanate of Oman. Al-Barwani, A., Helmi, T., 2006. Sea water intrusion in a coastal aquifer: a case study for the area between Seeb and Suwaiq, Sultanate of Oman. SQU Journal of Agricultural and Marine Sciences 11, 55–69. Ayers, R.S., Westcot, D.W., 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29. Food and Agriculture Organization of the United Nations, Rome, 174 pp. Castro, E., Manas, M.P., Heras, J., 2011. Effects of wastewater irrigation on soil properties and turfgrass growth. Water Science and Technology 63, 1678– 1688. Corwin, D.L., Rhoades, J.D., Simunek, J., 2007. Leaching requirement for soil salinity control: steady-state versus transient models. Agricultural Water Management 90 (3), 165–180. CSIRO Land and Water Home. Web. Water Use and Reuse Stream Projects.
234
S.A. Al Khamisi et al. / Agricultural Water Management 116 (2013) 228–234
Qadir, M., Sharma, B.R., Bruggeman, A., Choukr-Alla, R., Karajeh, F., 2007. Nonconventional water resources and opportunities for water augmentation to achieve food security in water scarce countries. Agricultural Water Management 87, 2–22. SDLAC (Sanitation Districts of Los Angeles County), 2011. Wastewater Facilities. SDLAC Website – Homepage. Web.
Tanji, K.K., Kielen, N.C., 2002. FAO Irrigation and Drainage Paper 61: Agricultural Drainage Water Management in Arid and Semi-Arid Areas. FAO, Rome. ISSN 0254-5264. West Basin Home. Web. Water Reliability 2020.