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Effect of shallow groundwater table on crop water requirements and crop yields
Agricultural Water Management 76 (2005) 24–35 www.elsevier.com/locate/agwat
Effect of shallow groundwater table on crop water requirements and crop yields M.A. Kahlown a, M. Ashraf b,*, Zia-ul-Haq c a b
Pakistan Council of Research in Water Resources, Khyaban-e-Johar Road H-8/1, Islamabad, Pakistan Pakistan Council of Research in Water Resources, Khyaban-e-Johar Road H-8/1, Islamabad, Pakistan c Pakistan Council of Research in Water Resources, 6.4 km Raiwind Road, Lahore, Pakistan Accepted 17 January 2005 Available online 16 March 2005
Abstract Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m 3.05 m 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water. # 2005 Elsevier B.V. All rights reserved. Keywords: Groundwater contribution; Crop water requirements; Crop yield; Salinity distribution; Wheat; Sugarcane; Maize; Sorghum; Berseem; Sunflower and Pakistan * Corresponding author. Tel.: +92 51 9258956; fax: +92 51 9258963. E-mail address: [email protected] (M. Ashraf). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2005.01.005
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1. Introduction Irrigated agriculture in the Indus basin is the major user of water in Pakistan. About 93% of the total water resources are used by agriculture (Latif, 2002). The gap between water demand and supply has increased manifolds, due to increased agricultural activities and reduced river flows. Availability of adequate good quality water is one of the most important inputs in successful crop production. Distribution of water among the canals in Pakistan is generally based on historical allocations and does not consider crop water requirements, water-table depth, and soil physico-chemical conditions. About a century ago, water allowances were fixed for different canals depending upon the surface water availability and the area to be covered. Since then many changes have taken place. Due to seepage from the irrigation network and non-functional drainage systems, water table in many areas had risen to near the soil surface. Rafique (1990) reported that in Pakistan 1.47 million hectars (Mha) has a water table within 1.5 m of the surface. Out of this, 0.13 Mha is covered by severely saline, uncultivated soils. In non-saline soils, 0.32 Mha has water table at 1.0–1.5 m, 0.28 Mha at 0.5–1.0 m depth and 0.74 Mha within 0.5 m. By the end of the dry season, 13% of the irrigated land had water tables less than 1.5 m from the surface. However, after the monsoon, 26% of the irrigated area had the water table less than 1.5 m (Qureshi and Barrett-Lennard, 1998). Groundwater is a flexible and reliable source of water. However, excessive pumping by deep public and private tubewells is often pulling up water with substantial salinity and is causing secondary soil salinization, whereas shallow fresh groundwater is not utilized. Therefore, there is a need for more judicious use of this precious water. Shallow groundwater could also be used as sub-irrigation by adopting proper irrigation scheduling to help bridge the gap between water demand and supply. Ayars and Schoneman (1986) showed that during 3 years of cotton growth, for a watertable depth of 1.7–2.1 m, with the water having an ECe = 10 dS m1, the evapotranspiration (ET) contributed by the groundwater ranged from 0 to a maximum of 37% whereas Wallender et al. (1979) found that cotton extracted 60% of its ET from a saline (EC = 6.0 dS m1) water table. Pratharpar and Qureshi (1998) observed that in areas where shallow water tables exist, the irrigation requirements can be reduced to 80% of the total crop ET without reducing crop yield and increasing soil salinization. This practice not only produced good yields but also kept the soil salinity and water-table depth within the acceptable limits. Kahlown et al. (1998) showed that there was an inverse relationship between the water-table depth and the groundwater contribution despite the brackish nature of the groundwater. They found that groundwater contribution was maximum at depth less than 1.0 m and was negligible when water-table depths exceeded 2 or 3 m. Soppe and Ayars (2003) studied the soil water fluxes in the presence of saline, shallow groundwater (EC = 14 dS m1) by maintaining water level at 1.5 m depth under a safflower crop and found groundwater contribution of up to 40% of daily crop water used. On a seasonal basis, 25% of the total crop water used originated from groundwater. The largest groundwater contribution occurred at the end of the growing season when roots were fully developed. The irrigation applied was 46% less when the water table was maintained at 1.5 m depth than when the water table was too deep to be reached by the crops.
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It is therefore obvious that the groundwater contribution is a significant component of water balance and should be recognized as providing part of the water needed by the crop for evapotranspiration. This will save water, energy, and labour and will also reduce the drainage effluent and help keep the water table at the desired depth. A series of studies were carried out in lysimeters to investigate the optimum utilization of water resources under shallow water-table conditions.
2. Objectives The main objectives of these studies were to determine (i) the irrigation requirements and evapotranspiration of various crops under different water-table depths, (ii) groundwater contribution to the crop water requirement under different water-tables depths, (iii) effect of different water table-depths on crop yields.
3. Material and methods In order to evaluate the contributions of groundwater to meet the water requirements of various crops, a study was initiated in Lahore Pakistan (318 340 N, 748 200 E) an area with arid climate (average maximum and minimum temperatures of 22.7 8C and 8.3 8C in winter, respectively; 36.5 8C and 24.7 8C in summer, respectively and with an average annual rainfall of 51 cm. The average rainfall during the cropping period of wheat, maize, berseem, sunflower, sugarcane and sorghum was 5, 34, 1.5, 0.6, 57 and 40 cm, respectively. The study was conducted over a period of 10 years. Eighteen large size drainage type concrete lysimeters of the size 3.05 m 3.05 m in area and 6.1 m deep were constructed. The soil profile consisted of two horizons. The top horizons were of silt loam texture extending from surface to 4.3 m depth. The bottom horizon was 0.9 m thick and was of loamy fine sand. A 0.9 m thick calcareous graded gravel filter was provided at the bottom of each lysimeter to facilitate the flow of water. The soil bulk density varied from 1.45–1.48 gm cm3 and was representative of the bulk density of the area. The lysimeters were surrounded by a field containing the same crops being grown in the lysimeters to avoid oasis effect. The mechanical analysis of the soil filled in the lysimeter is given in Table 1. The lysimeters were attached to bottles with Marriotte siphons, which maintained water level at desired depths. The water-table depths ranged from 0.5 to 3 m in 0.5 m increments. This resulted in six treatments with three replications and 18 lysimeters. The 3 m depth was Table 1 Mechanical analyses of the soil profile Depth (m)
0–4.3 4.3–5.2
Percentage of soil fractions Sand (%)
Silt (%)
Clay (%)
Classification
17 84
70 12
13 4
Silt loam Loamy fine sand
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assumed to be the depth with no groundwater contribution. For each irrigation, a fixed depth of water was applied and the excess amount of water percolated into the groundwater was measured through storage bottles attached to the bottom of each lysimeter and was subtracted when water balance was calculated. The groundwater contribution was calculated from the daily loss of water from the Marriotte bottles. Tubewell water (EC = 0.95 dS m1) was used for irrigation and to maintain water level in the Marriotte bottles. Soil water potential in the root zone was monitored daily by recording soil moisture tensions from the tensiometers. The tensiometers were installed at 20, 50, 75, 100, 125 and 150 cm from the soil surface. An irrigation of 7.62 cm was given to these crops when the soil moisture tension at the 20 cm depth reached at 60–65 k Pa. This amount was sufficient to replenish the field capacity. Generally under field conditions, it is not possible to apply irrigation less than 7.62 cm. After each effective rainfall, the soil moisture tension decreased thereby increasing the irrigation interval. The irrigation frequencies ranged from one in case of wheat, maize, berseem, sorghum and sunflower to more than 12 for sugarcane depending upon the rainfall and water-table conditions. All the tensiometers worked satisfactorily to the time of crop maturity. Occasionally when the tensiometers were under operation, air diffused into the tensiometers through the tensiometer cups, particularly at times of high soil moisture tension. The diffused air was removed immediately as soon as air bubbles were seen in the tensiometers. Fig. 1 shows the schematic diagram of the lysimeter. At site, the meteorological data such as maximum and minimum temperature with maximum and minimum thermometer, relative humidity with wet and dry bulb, wind speed with anemometer, pan evaporation with class A pan and rainfall with standard rain gauge were recorded regularly. The recommended fertilizers and insecticides doses were applied. The crops studied in the lysimeters were wheat, maize, sugarcane, sunflower, berseem and
Fig. 1. Schematic diagram of a lysimeter.
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sorghum. Each crop was replicated thrice in a complete randomized design. Actual evapotranspiration of the crop was computed using the water balance equation: ET ¼ I þ S þ R D DSM where ET represents evapotranspiration, I, S, R, D, and DSM denote irrigation, groundwater contribution, rainfall, drainage surplus and changes in soil moisture storage, respectively. The reference evapotranspiration (ETo) was determined with Blenny–Criddle method to compute the crop coefficients. The data presented in the results and discussion is the average of three replications. Tensiometer data were used to determine changes in moisture storage through a predetermined moisture-retention curve (Fig. 2). The crops were harvested from whole plot at the time of maturity and the air-dried grain yields were determined. In case of sugarcane, the yield was determined after removing leaves. Table 2 shows salient features of the crops studied.
4. Results and discussion The results of these studies are presented and discussed in the following sections. 4.1. Effect of water-table depth on irrigation requirements of crops The water application data (Fig. 3) showed that for wheat, the irrigation requirement was minimum (less than 5 cm) at 1.0 m water-table depth and increased with increase in water-table depth. For maize, the irrigation requirement with the water table at 1.0 m depth, was about 7.5 cm which also increased with increase in water table depth. This was mainly due to the fact that at shallower water-table depths, groundwater contributions were higher which consequently reduced the irrigation requirements. The irrigation requirements decreased almost in a linear fashion with an increase in water-table depth for all the crops except for sorghum. Sugarcane and berseem had almost the same irrigation requirement under the same water-table depths. The sugarcane extracted more water at 1.0 m depth, probably due to its long roots. The irrigation requirement of sugarcane reduced drastically with decreasing water-table depth indicating that water from the shallow water table was contributing significantly to satisfying crop water requirement. However, there was no effect of water-table depth on irrigation requirement of sorghum and its crop water
Fig. 2. Moisture-retention curve.
Name of crop
Botanical name
Rooting depth (cm)
Sowing time
Harvesting time
Remarks
Wheat Berseem
Triticum aestivum Trifolium alexandrium juslen
90–150 50–100
October/November September
March/April March
Sorghum
Sorghum bicolor
200
June
August
Maize Sunflower Sugarcane
Zea mays Helianthus annuus Saccharum officinarum
90–150 50–100 60–150
July January/February September/October
October April/May November/December
Salt tolerant crop Moderately sensitive to soil salinity and can tolerate higher level of moisture Drought resistant crop, can tolerate short period of water logging and is moderately tolerant to soil salinity Sensitive to excess water Tap root system Has extensive fibrous root system in the upper 60–90 cm of soil. Some roots may extend upto 240 cm depth
Source: On Farm Water Management Field Manual (1997).
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Table 2 Salient features of the crops grown
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Fig. 3. Irrigation water requirements as a function of water-table depth.
requirement was minimum as compared to the other crops studied. The irrigation requirement of sunflower was much greater than that of wheat. The differences in irrigation requirements under each crop and water-table depth, can be attributed mainly to greater absorption of water from shallow water tables, variation of rooting extent of the crops, and greater abilities of roots of some crops to live near and even below the water table.
4.2. Effect of water-table depth on groundwater contribution Groundwater contribution was the highest under the shallowest watertable conditions, which gradually reduced with increasing water-table depth (Fig. 4). Wheat was able to take up more than 90% of its water from the groundwater and sunflower was able to take up more than 80% of its water from the groundwater when the water table depth was at 0.5 m. However, for sugarcane, berseem and sorghum, the groundwater contribution at 0.5 m depth could not be determined because these crops could not survive at this water table
Fig. 4. Groundwater contribution as a function of water-table depth.
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depth. Under shallow water table conditions, Kahlown et al. (2000) reported that on a loam soil with a water table at less than 1 m deep, wheat and sugarcane could obtain 32 and 53% of their water needs, respectively. As indicated in Fig. 4, maize was able to obtain about 40% of its water needs when the water table was at 0.5 m depth. Sugarcane and berseem extracted 50 and 30%, respectively of their total water needs from groundwater when water tables were at 1.0 m depth. Sorghum utilized groundwater to fill only 10% of its water need when the water table was at 1.0 m depth. With increase in water-table depth, the groundwater contribution decreased in all crops. Ayars et al. (1999) reported that extending the interval between irrigation applications would increase groundwater use under shallow water-table conditions. 4.3. Effect of water-table depth on evapotranspiration (ET) of crops The differences in ET between crops are due primarily to differences in length and season crops grown and phenology of crops. The evapotranspiration was the highest at 0.5 m water table depth for wheat and sunflower (Fig. 5). It slightly decreased with increase in water-table depth for these two crops and attained a minimum value at about 1.50 m depth. Some of the other crops may have had maximum when the water table was at 0.5 m if they had been growing under that condition. The rate of evaporation from the soil surface depends upon climatic factors, the unsaturated hydraulic conductivity, moisture content of the soil, concentration of salts in the soil groundwater and the depth to the water-table. When the water table is near the surface, the water contents of the surface soil remain high due to capillary flux equalling the high evaporative flux. These wet surfaces evaporate water rapidly. In case of comparatively deep water-table conditions, the capillary action does not carry water to the soil surface as fast as evaporation takes it away. Due to this drying of the soil surface, evaporation from the soil surface decreases to low values and ET reaches to minimums. In case of very deep water tables where the groundwater is supplying little water, the crops were irrigated more frequently. During and for several hours following each irrigation, the soil surface is wet and if there is not complete canopy cover, rapid evaporation takes place until the surface dries. These more frequent wetting and
Fig. 5. Evapotranspiration as a function of water-table depth.
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drying cycles when water tables are deep are probably the cause of trend to slightly higher ET when water tables dropped below the reach of the roots. Being a 12-month crop, the total ET of sugarcane was higher than the other crops (more than 150 cm). The total ET of sorghum, which grows actively for only about three months was less than 35 cm. Maize showed a tendency for increased ET with increased water table depth. This could be due to increased number of irrigations and increased evaporation discussed previously. 4.4. Monthly crop co-efficient for crops The effect of crop characteristics on crop water requirements is accounted for by the crop co-efficient (Kc), which is used to relate the reference ETo to the actual crop evapotranspiration of a crop under optimum soil moisture. Monthly crop coefficients of crops have been presented in Fig. 6. It is obvious from the figure that, in general Kc value was the lowest in the first month of crop establishment and gradually increased and attained a peak value at grain formation stage. The Kc values dropped down by about 50–60% at the time of crop maturity. Sugarcane had maximum Kc values during May–June and the lowest during September–October. For berseem however, the Kc value increased gradually. It maintained peak value until the crop was harvested. 4.5. Effect of water-table depth on crop yields The crop growth and subsequently the yield primarily depend on the favourable environment in the root zone, rooting depth, sensitivity of crop for water etc. The effects of different water-table depths on crop yield are shown in Figs. 7 and 8. Wheat yield was maximum (5.5 T/ha) at 1.5 m depth (Fig. 7). With the water tables, below and above this level, wheat yield was reduced. This reduction however, was more pronounced at 0.5 m depth probably due to reduced aeration in the root zone. The other crops followed the same trend of yield reduction when water tables were less than 1.5 m. Berseem, sugarcane and sunflower yields were even better when the water table was 2 m or more below the surface.
Fig. 6. Monthly crop co-efficients (Kc) for various crops.
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Fig. 7. Crop yields as a function of water-table depth.
Fig. 8. Effect of water table depth on sugarcane yield.
The slight reduction in wheat yield below 1.5 m may be attributed to less availability of water for crop use but it was not significant (Table 3). Kahlown et al. (1998) found a linear relationship between the wheat yield and water-table depth. They obtained optimum yield of wheat with 22.5 cm of irrigation water when the water-table depth was less than 1 m whereas 30.5 cm of irrigation water was required when the water-table depths ranges from 1 to 2 m. Asad (2001) also concluded that high wheat yield could be obtained with 1–2 irrigations of 7.5 cm each when the water table depth was from 1 to 2 m. However, if the groundwater is saline generally more than 4.0 dS m1, the water table of 1–2 m or less results in decreased wheat and sugarcane yields (Kahlown and Azam, 2002). Maximum sugarcane yield was obtained with water table at 2 m or below with drastic reduction in yield when water table was lowered from 2 m (Fig. 8). Mejia et al. (2000) conducted a two year study to evaluate the effect of water table on the yields of corn. The treatments consisted of water table at 0.5 and 0.75 m depth and compared the results with the control (free drainage, water table 1.0 m below the soil surface). On an average, they found 5–10% greater yields for corn and 23% for soybean over control. They recommended a water-table level of 0.75 cm for corn and soybean production. In the present study however, wheat maize, sunflower, sugarcane and berseem gave maximum yields at 1.5 m or greater water-table depth. It is therefore concluded that water-table depth of 1.5–2.0 m is the optimum water table depth for most crops except sugarcane which gave
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Table 3 The effect different water-table depths on crop yield Treatment
Significant difference (at 5% significant level) Wheat
Maize
Sunflower
Berseem
Sorghum
Sugarcane
T1 T1 T1 T1 T2 T2 T2 T3 T3 T4
S S S S N.S N.S N.S N.S N.S N.S
S S S S S S S N.S N.S N.S
S S S S S S S N.S N.S N.S
– – – – S S S S S S
– – – – S S S N.S S N.S
– – – – S S S S S N.S
& & & & & & & & & &
T2 T3 T4 T5 T3 T4 T5 T4 T5 T5
NB: T1, T2, T3, T4, T5 are the water-table depths at 0.5, 1.0, 1.5, 2 and 3 m, respectively. S: significant, N.S: non significant.
even higher yields with the water table at 2 m depths. Asad (2001) recommended 1–1.5 m water table depth for optimum wheat and cotton yields. 5. Conclusions In areas with shallow water table (generally less than 3 m), some crops can draw water from the groundwater, crop yields can often be enhanced and the amount of irrigation applied can be reduced significantly. Under very shallow water-table conditions (0.5 m depth), wheat extracted almost all its required water from the groundwater whereas sunflower extracted more than 80% of its requirement. It was concluded that 1.5–2.0 m was the optimum watertable depth for all the crops studied. The present system of allocation of irrigation supplies, especially in the areas where water table is shallow, needs modification to avoid inefficient use of water. When the water table is less than 2 m below the soil surface, most crops can obtain a substantial portion of their needed water from the groundwater and consequently irrigation rates can be reduced below the evapotranspiration rates. Acknowledgements The authors would like to thank Ch. Talib Ali, Ex- Director, Mr. Abdur Raoof Regional Director and Mr. Noor Ullah, Field Assistant, Pakistan Council of Research in Water Resources Lahore for their help in data collection. The authors are also thankful to Dr. W.D. Kemper for reviewing the manuscript.
References Asad, S.Q., 2001. Irrigation requirements of wheat and cotton under different water-table conditions. Sarhad J. Agric. 17, 7–13.
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Ayars, J.E., Schoneman, R.A., 1986. Use of saline water from a shallow water table by cotton. Trans ASAE 29, 1674–1678. Ayars, J.E., Hutmacher, R.B., Schoneman, R.A., Soppe, R.W.O., Vail, F., Dale, S.S., 1999. Realizing the potential of integrated irrigation and drainage water management for meeting crop water requirements in semi-arid and arid areas. Irrig. Drain. Syst. 13, 321–347. Kahlown, M.A., Iqbal, M., Skogerboe, G.V., Rehman S.U., 1998. Water logging, salinity and crop yield relationships. Mona Reclamation Experimental Project, WAPDA, Report No. 233. Kahlown, M.A., Raoof, A., Junejo, M.R., Hanif, M., 2000. Effect of shallow drainage on crop water requirements. In: Chandio, S.N. (Ed.), Proceedings of National Seminar on Drainage in Pakistan, August 16–18 2000, Jamshoro. Kahlown, M.A., Azam, M., 2002. Individual and combined effect of waterlogging and salinity on crop yields in the Indus basin. Irrig. Drain. 51, 329–338. Latif, M., 2002. Measures to mitigate impacts of drought: global perspective and conditions in Pakistan. In: Saeed, M.M., Latif, M. (Eds.), Proceedings of the National Symposium on Drought and Water Resources in Pakistan, March 18, 2002, CEWRE, Lahore, Pakistan, pp. 1–4. Mejia, M.N., Madramootoo, C.A., Broughton, R.S., 2000. Influence of water table management on corn and soybean yields. Agri. Water Manage. 46, 73–89. On Farm Water Management Field Manual, 1997. On Farm Water Management Field Manual: Volume VI Irrigation Agronomy. Federal Water Management Cell, Ministry of Food, Agriculture and Livestock, Government of Pakistan, Islamabad. Pratharpar, S.A., Qureshi, A.S., 1998. Modelling the effects of deficit irrigation on soil salinity, depth to water table and transpiration in semi-arid zones with monsoonal rains. Water Resour. Dev. 15, 141–159. Qureshi, R.H., Barrett-Lennard, E.G., 1998. Saline Agriculture for Irrigated Land in Pakistan: A handbook. Australian Centre for International Agriculture Research, Canberra, Australia. Rafique, M., 1990. Soil resources and soil related problems in Pakistan. In: Mumtaz, A., Ehsan, M., Nizami, M.I. (Eds.), Proceedings of the International Symposium on Applied Soil Physics in Stress Environments, 22–26 January 1989, Islamabad, Pakistan, pp. 16–23. Soppe, R.W.O., Ayars, J.E., 2003. Characterizing groundwater use by safflower using lysimeters. Agri. Water Manage. 60, 59–71. Wallender, W.W., Grimes, D.W., Henderson, D.W., Stromberg, S.K., 1979. Estimating the contribution of a perched water table to the seasonal evapotranspiration of cotton. Agron. J. 71, 1056–1060.
Effect of shallow groundwater table on crop water requirements and crop yields M.A. Kahlown a, M. Ashraf b,*, Zia-ul-Haq c a b
Pakistan Council of Research in Water Resources, Khyaban-e-Johar Road H-8/1, Islamabad, Pakistan Pakistan Council of Research in Water Resources, Khyaban-e-Johar Road H-8/1, Islamabad, Pakistan c Pakistan Council of Research in Water Resources, 6.4 km Raiwind Road, Lahore, Pakistan Accepted 17 January 2005 Available online 16 March 2005
Abstract Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m 3.05 m 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water. # 2005 Elsevier B.V. All rights reserved. Keywords: Groundwater contribution; Crop water requirements; Crop yield; Salinity distribution; Wheat; Sugarcane; Maize; Sorghum; Berseem; Sunflower and Pakistan * Corresponding author. Tel.: +92 51 9258956; fax: +92 51 9258963. E-mail address: [email protected] (M. Ashraf). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2005.01.005
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1. Introduction Irrigated agriculture in the Indus basin is the major user of water in Pakistan. About 93% of the total water resources are used by agriculture (Latif, 2002). The gap between water demand and supply has increased manifolds, due to increased agricultural activities and reduced river flows. Availability of adequate good quality water is one of the most important inputs in successful crop production. Distribution of water among the canals in Pakistan is generally based on historical allocations and does not consider crop water requirements, water-table depth, and soil physico-chemical conditions. About a century ago, water allowances were fixed for different canals depending upon the surface water availability and the area to be covered. Since then many changes have taken place. Due to seepage from the irrigation network and non-functional drainage systems, water table in many areas had risen to near the soil surface. Rafique (1990) reported that in Pakistan 1.47 million hectars (Mha) has a water table within 1.5 m of the surface. Out of this, 0.13 Mha is covered by severely saline, uncultivated soils. In non-saline soils, 0.32 Mha has water table at 1.0–1.5 m, 0.28 Mha at 0.5–1.0 m depth and 0.74 Mha within 0.5 m. By the end of the dry season, 13% of the irrigated land had water tables less than 1.5 m from the surface. However, after the monsoon, 26% of the irrigated area had the water table less than 1.5 m (Qureshi and Barrett-Lennard, 1998). Groundwater is a flexible and reliable source of water. However, excessive pumping by deep public and private tubewells is often pulling up water with substantial salinity and is causing secondary soil salinization, whereas shallow fresh groundwater is not utilized. Therefore, there is a need for more judicious use of this precious water. Shallow groundwater could also be used as sub-irrigation by adopting proper irrigation scheduling to help bridge the gap between water demand and supply. Ayars and Schoneman (1986) showed that during 3 years of cotton growth, for a watertable depth of 1.7–2.1 m, with the water having an ECe = 10 dS m1, the evapotranspiration (ET) contributed by the groundwater ranged from 0 to a maximum of 37% whereas Wallender et al. (1979) found that cotton extracted 60% of its ET from a saline (EC = 6.0 dS m1) water table. Pratharpar and Qureshi (1998) observed that in areas where shallow water tables exist, the irrigation requirements can be reduced to 80% of the total crop ET without reducing crop yield and increasing soil salinization. This practice not only produced good yields but also kept the soil salinity and water-table depth within the acceptable limits. Kahlown et al. (1998) showed that there was an inverse relationship between the water-table depth and the groundwater contribution despite the brackish nature of the groundwater. They found that groundwater contribution was maximum at depth less than 1.0 m and was negligible when water-table depths exceeded 2 or 3 m. Soppe and Ayars (2003) studied the soil water fluxes in the presence of saline, shallow groundwater (EC = 14 dS m1) by maintaining water level at 1.5 m depth under a safflower crop and found groundwater contribution of up to 40% of daily crop water used. On a seasonal basis, 25% of the total crop water used originated from groundwater. The largest groundwater contribution occurred at the end of the growing season when roots were fully developed. The irrigation applied was 46% less when the water table was maintained at 1.5 m depth than when the water table was too deep to be reached by the crops.
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It is therefore obvious that the groundwater contribution is a significant component of water balance and should be recognized as providing part of the water needed by the crop for evapotranspiration. This will save water, energy, and labour and will also reduce the drainage effluent and help keep the water table at the desired depth. A series of studies were carried out in lysimeters to investigate the optimum utilization of water resources under shallow water-table conditions.
2. Objectives The main objectives of these studies were to determine (i) the irrigation requirements and evapotranspiration of various crops under different water-table depths, (ii) groundwater contribution to the crop water requirement under different water-tables depths, (iii) effect of different water table-depths on crop yields.
3. Material and methods In order to evaluate the contributions of groundwater to meet the water requirements of various crops, a study was initiated in Lahore Pakistan (318 340 N, 748 200 E) an area with arid climate (average maximum and minimum temperatures of 22.7 8C and 8.3 8C in winter, respectively; 36.5 8C and 24.7 8C in summer, respectively and with an average annual rainfall of 51 cm. The average rainfall during the cropping period of wheat, maize, berseem, sunflower, sugarcane and sorghum was 5, 34, 1.5, 0.6, 57 and 40 cm, respectively. The study was conducted over a period of 10 years. Eighteen large size drainage type concrete lysimeters of the size 3.05 m 3.05 m in area and 6.1 m deep were constructed. The soil profile consisted of two horizons. The top horizons were of silt loam texture extending from surface to 4.3 m depth. The bottom horizon was 0.9 m thick and was of loamy fine sand. A 0.9 m thick calcareous graded gravel filter was provided at the bottom of each lysimeter to facilitate the flow of water. The soil bulk density varied from 1.45–1.48 gm cm3 and was representative of the bulk density of the area. The lysimeters were surrounded by a field containing the same crops being grown in the lysimeters to avoid oasis effect. The mechanical analysis of the soil filled in the lysimeter is given in Table 1. The lysimeters were attached to bottles with Marriotte siphons, which maintained water level at desired depths. The water-table depths ranged from 0.5 to 3 m in 0.5 m increments. This resulted in six treatments with three replications and 18 lysimeters. The 3 m depth was Table 1 Mechanical analyses of the soil profile Depth (m)
0–4.3 4.3–5.2
Percentage of soil fractions Sand (%)
Silt (%)
Clay (%)
Classification
17 84
70 12
13 4
Silt loam Loamy fine sand
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assumed to be the depth with no groundwater contribution. For each irrigation, a fixed depth of water was applied and the excess amount of water percolated into the groundwater was measured through storage bottles attached to the bottom of each lysimeter and was subtracted when water balance was calculated. The groundwater contribution was calculated from the daily loss of water from the Marriotte bottles. Tubewell water (EC = 0.95 dS m1) was used for irrigation and to maintain water level in the Marriotte bottles. Soil water potential in the root zone was monitored daily by recording soil moisture tensions from the tensiometers. The tensiometers were installed at 20, 50, 75, 100, 125 and 150 cm from the soil surface. An irrigation of 7.62 cm was given to these crops when the soil moisture tension at the 20 cm depth reached at 60–65 k Pa. This amount was sufficient to replenish the field capacity. Generally under field conditions, it is not possible to apply irrigation less than 7.62 cm. After each effective rainfall, the soil moisture tension decreased thereby increasing the irrigation interval. The irrigation frequencies ranged from one in case of wheat, maize, berseem, sorghum and sunflower to more than 12 for sugarcane depending upon the rainfall and water-table conditions. All the tensiometers worked satisfactorily to the time of crop maturity. Occasionally when the tensiometers were under operation, air diffused into the tensiometers through the tensiometer cups, particularly at times of high soil moisture tension. The diffused air was removed immediately as soon as air bubbles were seen in the tensiometers. Fig. 1 shows the schematic diagram of the lysimeter. At site, the meteorological data such as maximum and minimum temperature with maximum and minimum thermometer, relative humidity with wet and dry bulb, wind speed with anemometer, pan evaporation with class A pan and rainfall with standard rain gauge were recorded regularly. The recommended fertilizers and insecticides doses were applied. The crops studied in the lysimeters were wheat, maize, sugarcane, sunflower, berseem and
Fig. 1. Schematic diagram of a lysimeter.
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sorghum. Each crop was replicated thrice in a complete randomized design. Actual evapotranspiration of the crop was computed using the water balance equation: ET ¼ I þ S þ R D DSM where ET represents evapotranspiration, I, S, R, D, and DSM denote irrigation, groundwater contribution, rainfall, drainage surplus and changes in soil moisture storage, respectively. The reference evapotranspiration (ETo) was determined with Blenny–Criddle method to compute the crop coefficients. The data presented in the results and discussion is the average of three replications. Tensiometer data were used to determine changes in moisture storage through a predetermined moisture-retention curve (Fig. 2). The crops were harvested from whole plot at the time of maturity and the air-dried grain yields were determined. In case of sugarcane, the yield was determined after removing leaves. Table 2 shows salient features of the crops studied.
4. Results and discussion The results of these studies are presented and discussed in the following sections. 4.1. Effect of water-table depth on irrigation requirements of crops The water application data (Fig. 3) showed that for wheat, the irrigation requirement was minimum (less than 5 cm) at 1.0 m water-table depth and increased with increase in water-table depth. For maize, the irrigation requirement with the water table at 1.0 m depth, was about 7.5 cm which also increased with increase in water table depth. This was mainly due to the fact that at shallower water-table depths, groundwater contributions were higher which consequently reduced the irrigation requirements. The irrigation requirements decreased almost in a linear fashion with an increase in water-table depth for all the crops except for sorghum. Sugarcane and berseem had almost the same irrigation requirement under the same water-table depths. The sugarcane extracted more water at 1.0 m depth, probably due to its long roots. The irrigation requirement of sugarcane reduced drastically with decreasing water-table depth indicating that water from the shallow water table was contributing significantly to satisfying crop water requirement. However, there was no effect of water-table depth on irrigation requirement of sorghum and its crop water
Fig. 2. Moisture-retention curve.
Name of crop
Botanical name
Rooting depth (cm)
Sowing time
Harvesting time
Remarks
Wheat Berseem
Triticum aestivum Trifolium alexandrium juslen
90–150 50–100
October/November September
March/April March
Sorghum
Sorghum bicolor
200
June
August
Maize Sunflower Sugarcane
Zea mays Helianthus annuus Saccharum officinarum
90–150 50–100 60–150
July January/February September/October
October April/May November/December
Salt tolerant crop Moderately sensitive to soil salinity and can tolerate higher level of moisture Drought resistant crop, can tolerate short period of water logging and is moderately tolerant to soil salinity Sensitive to excess water Tap root system Has extensive fibrous root system in the upper 60–90 cm of soil. Some roots may extend upto 240 cm depth
Source: On Farm Water Management Field Manual (1997).
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Table 2 Salient features of the crops grown
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Fig. 3. Irrigation water requirements as a function of water-table depth.
requirement was minimum as compared to the other crops studied. The irrigation requirement of sunflower was much greater than that of wheat. The differences in irrigation requirements under each crop and water-table depth, can be attributed mainly to greater absorption of water from shallow water tables, variation of rooting extent of the crops, and greater abilities of roots of some crops to live near and even below the water table.
4.2. Effect of water-table depth on groundwater contribution Groundwater contribution was the highest under the shallowest watertable conditions, which gradually reduced with increasing water-table depth (Fig. 4). Wheat was able to take up more than 90% of its water from the groundwater and sunflower was able to take up more than 80% of its water from the groundwater when the water table depth was at 0.5 m. However, for sugarcane, berseem and sorghum, the groundwater contribution at 0.5 m depth could not be determined because these crops could not survive at this water table
Fig. 4. Groundwater contribution as a function of water-table depth.
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depth. Under shallow water table conditions, Kahlown et al. (2000) reported that on a loam soil with a water table at less than 1 m deep, wheat and sugarcane could obtain 32 and 53% of their water needs, respectively. As indicated in Fig. 4, maize was able to obtain about 40% of its water needs when the water table was at 0.5 m depth. Sugarcane and berseem extracted 50 and 30%, respectively of their total water needs from groundwater when water tables were at 1.0 m depth. Sorghum utilized groundwater to fill only 10% of its water need when the water table was at 1.0 m depth. With increase in water-table depth, the groundwater contribution decreased in all crops. Ayars et al. (1999) reported that extending the interval between irrigation applications would increase groundwater use under shallow water-table conditions. 4.3. Effect of water-table depth on evapotranspiration (ET) of crops The differences in ET between crops are due primarily to differences in length and season crops grown and phenology of crops. The evapotranspiration was the highest at 0.5 m water table depth for wheat and sunflower (Fig. 5). It slightly decreased with increase in water-table depth for these two crops and attained a minimum value at about 1.50 m depth. Some of the other crops may have had maximum when the water table was at 0.5 m if they had been growing under that condition. The rate of evaporation from the soil surface depends upon climatic factors, the unsaturated hydraulic conductivity, moisture content of the soil, concentration of salts in the soil groundwater and the depth to the water-table. When the water table is near the surface, the water contents of the surface soil remain high due to capillary flux equalling the high evaporative flux. These wet surfaces evaporate water rapidly. In case of comparatively deep water-table conditions, the capillary action does not carry water to the soil surface as fast as evaporation takes it away. Due to this drying of the soil surface, evaporation from the soil surface decreases to low values and ET reaches to minimums. In case of very deep water tables where the groundwater is supplying little water, the crops were irrigated more frequently. During and for several hours following each irrigation, the soil surface is wet and if there is not complete canopy cover, rapid evaporation takes place until the surface dries. These more frequent wetting and
Fig. 5. Evapotranspiration as a function of water-table depth.
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drying cycles when water tables are deep are probably the cause of trend to slightly higher ET when water tables dropped below the reach of the roots. Being a 12-month crop, the total ET of sugarcane was higher than the other crops (more than 150 cm). The total ET of sorghum, which grows actively for only about three months was less than 35 cm. Maize showed a tendency for increased ET with increased water table depth. This could be due to increased number of irrigations and increased evaporation discussed previously. 4.4. Monthly crop co-efficient for crops The effect of crop characteristics on crop water requirements is accounted for by the crop co-efficient (Kc), which is used to relate the reference ETo to the actual crop evapotranspiration of a crop under optimum soil moisture. Monthly crop coefficients of crops have been presented in Fig. 6. It is obvious from the figure that, in general Kc value was the lowest in the first month of crop establishment and gradually increased and attained a peak value at grain formation stage. The Kc values dropped down by about 50–60% at the time of crop maturity. Sugarcane had maximum Kc values during May–June and the lowest during September–October. For berseem however, the Kc value increased gradually. It maintained peak value until the crop was harvested. 4.5. Effect of water-table depth on crop yields The crop growth and subsequently the yield primarily depend on the favourable environment in the root zone, rooting depth, sensitivity of crop for water etc. The effects of different water-table depths on crop yield are shown in Figs. 7 and 8. Wheat yield was maximum (5.5 T/ha) at 1.5 m depth (Fig. 7). With the water tables, below and above this level, wheat yield was reduced. This reduction however, was more pronounced at 0.5 m depth probably due to reduced aeration in the root zone. The other crops followed the same trend of yield reduction when water tables were less than 1.5 m. Berseem, sugarcane and sunflower yields were even better when the water table was 2 m or more below the surface.
Fig. 6. Monthly crop co-efficients (Kc) for various crops.
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Fig. 7. Crop yields as a function of water-table depth.
Fig. 8. Effect of water table depth on sugarcane yield.
The slight reduction in wheat yield below 1.5 m may be attributed to less availability of water for crop use but it was not significant (Table 3). Kahlown et al. (1998) found a linear relationship between the wheat yield and water-table depth. They obtained optimum yield of wheat with 22.5 cm of irrigation water when the water-table depth was less than 1 m whereas 30.5 cm of irrigation water was required when the water-table depths ranges from 1 to 2 m. Asad (2001) also concluded that high wheat yield could be obtained with 1–2 irrigations of 7.5 cm each when the water table depth was from 1 to 2 m. However, if the groundwater is saline generally more than 4.0 dS m1, the water table of 1–2 m or less results in decreased wheat and sugarcane yields (Kahlown and Azam, 2002). Maximum sugarcane yield was obtained with water table at 2 m or below with drastic reduction in yield when water table was lowered from 2 m (Fig. 8). Mejia et al. (2000) conducted a two year study to evaluate the effect of water table on the yields of corn. The treatments consisted of water table at 0.5 and 0.75 m depth and compared the results with the control (free drainage, water table 1.0 m below the soil surface). On an average, they found 5–10% greater yields for corn and 23% for soybean over control. They recommended a water-table level of 0.75 cm for corn and soybean production. In the present study however, wheat maize, sunflower, sugarcane and berseem gave maximum yields at 1.5 m or greater water-table depth. It is therefore concluded that water-table depth of 1.5–2.0 m is the optimum water table depth for most crops except sugarcane which gave
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Table 3 The effect different water-table depths on crop yield Treatment
Significant difference (at 5% significant level) Wheat
Maize
Sunflower
Berseem
Sorghum
Sugarcane
T1 T1 T1 T1 T2 T2 T2 T3 T3 T4
S S S S N.S N.S N.S N.S N.S N.S
S S S S S S S N.S N.S N.S
S S S S S S S N.S N.S N.S
– – – – S S S S S S
– – – – S S S N.S S N.S
– – – – S S S S S N.S
& & & & & & & & & &
T2 T3 T4 T5 T3 T4 T5 T4 T5 T5
NB: T1, T2, T3, T4, T5 are the water-table depths at 0.5, 1.0, 1.5, 2 and 3 m, respectively. S: significant, N.S: non significant.
even higher yields with the water table at 2 m depths. Asad (2001) recommended 1–1.5 m water table depth for optimum wheat and cotton yields. 5. Conclusions In areas with shallow water table (generally less than 3 m), some crops can draw water from the groundwater, crop yields can often be enhanced and the amount of irrigation applied can be reduced significantly. Under very shallow water-table conditions (0.5 m depth), wheat extracted almost all its required water from the groundwater whereas sunflower extracted more than 80% of its requirement. It was concluded that 1.5–2.0 m was the optimum watertable depth for all the crops studied. The present system of allocation of irrigation supplies, especially in the areas where water table is shallow, needs modification to avoid inefficient use of water. When the water table is less than 2 m below the soil surface, most crops can obtain a substantial portion of their needed water from the groundwater and consequently irrigation rates can be reduced below the evapotranspiration rates. Acknowledgements The authors would like to thank Ch. Talib Ali, Ex- Director, Mr. Abdur Raoof Regional Director and Mr. Noor Ullah, Field Assistant, Pakistan Council of Research in Water Resources Lahore for their help in data collection. The authors are also thankful to Dr. W.D. Kemper for reviewing the manuscript.
References Asad, S.Q., 2001. Irrigation requirements of wheat and cotton under different water-table conditions. Sarhad J. Agric. 17, 7–13.
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35
Ayars, J.E., Schoneman, R.A., 1986. Use of saline water from a shallow water table by cotton. Trans ASAE 29, 1674–1678. Ayars, J.E., Hutmacher, R.B., Schoneman, R.A., Soppe, R.W.O., Vail, F., Dale, S.S., 1999. Realizing the potential of integrated irrigation and drainage water management for meeting crop water requirements in semi-arid and arid areas. Irrig. Drain. Syst. 13, 321–347. Kahlown, M.A., Iqbal, M., Skogerboe, G.V., Rehman S.U., 1998. Water logging, salinity and crop yield relationships. Mona Reclamation Experimental Project, WAPDA, Report No. 233. Kahlown, M.A., Raoof, A., Junejo, M.R., Hanif, M., 2000. Effect of shallow drainage on crop water requirements. In: Chandio, S.N. (Ed.), Proceedings of National Seminar on Drainage in Pakistan, August 16–18 2000, Jamshoro. Kahlown, M.A., Azam, M., 2002. Individual and combined effect of waterlogging and salinity on crop yields in the Indus basin. Irrig. Drain. 51, 329–338. Latif, M., 2002. Measures to mitigate impacts of drought: global perspective and conditions in Pakistan. In: Saeed, M.M., Latif, M. (Eds.), Proceedings of the National Symposium on Drought and Water Resources in Pakistan, March 18, 2002, CEWRE, Lahore, Pakistan, pp. 1–4. Mejia, M.N., Madramootoo, C.A., Broughton, R.S., 2000. Influence of water table management on corn and soybean yields. Agri. Water Manage. 46, 73–89. On Farm Water Management Field Manual, 1997. On Farm Water Management Field Manual: Volume VI Irrigation Agronomy. Federal Water Management Cell, Ministry of Food, Agriculture and Livestock, Government of Pakistan, Islamabad. Pratharpar, S.A., Qureshi, A.S., 1998. Modelling the effects of deficit irrigation on soil salinity, depth to water table and transpiration in semi-arid zones with monsoonal rains. Water Resour. Dev. 15, 141–159. Qureshi, R.H., Barrett-Lennard, E.G., 1998. Saline Agriculture for Irrigated Land in Pakistan: A handbook. Australian Centre for International Agriculture Research, Canberra, Australia. Rafique, M., 1990. Soil resources and soil related problems in Pakistan. In: Mumtaz, A., Ehsan, M., Nizami, M.I. (Eds.), Proceedings of the International Symposium on Applied Soil Physics in Stress Environments, 22–26 January 1989, Islamabad, Pakistan, pp. 16–23. Soppe, R.W.O., Ayars, J.E., 2003. Characterizing groundwater use by safflower using lysimeters. Agri. Water Manage. 60, 59–71. Wallender, W.W., Grimes, D.W., Henderson, D.W., Stromberg, S.K., 1979. Estimating the contribution of a perched water table to the seasonal evapotranspiration of cotton. Agron. J. 71, 1056–1060.