Field level
The monthly and seasonal recharge rates were determined by the water balance model at field level for the S-SL, M-SL and D-SCL hydrological response units for different crops (Table 4.6). The maximum values for the rice fields are due to percolation from the permanently flooded fields (Table 4.3). Due to the continuous flooding of the rice fields during the whole rice growing season, the recharge rates for different months do not differ much. GW contribution also has no influence on rice fields, as the fields are always flooded, thus the higher infiltration amounts suppressed the capillary rise (Thomas, 2003). Recharge rates are also high for perennial plants and cotton. For cotton, the rates are almost similar on M-SL and D-SCL soils. Although the seasonal capillary rise difference between S-SL and M-SL is 24 mm, the monthly variation of capillary rise caused the same seasonal recharge rates (Table 4.5). The capillary rise contribution during the month of August (month with high cotton crop water requirement) for S-SL is less than for M-SL, which increased the recharge rates for S- SL as compared to M-SL and eventually resulted in the same seasonal values. Higher recharge rates for perennial plants as compared to cotton are due to the long irrigation Month irrigation *Gross
mm
Rainfall mm
**Capillary rise (mm)
Cotton Wheat Vegetables
S-SL M-SL D-SCL S-SL M-SL D-SCL S-SL M-SL D-SCL April 75 41 7 9 0 4 9 0 1 0 0 May 105 4 54 32 13 24 6 0 17 3 0 June 270 9 43 20 32 6 5 0 12 6 0 July 355 1 49 51 34 0 0 0 17 11 0 August 419 0 7 28 14 0 0 0 0 0 0 September 65 0 4 0 4 0 0 0 0 0 0 Seasonal 1289 55 164 140 97 34 20 0 47 20 0
lower than for cotton during the peak irrigation season, higher irrigation requirements for the perennial plants caused the higher seasonal recharge rates.
Seasonal recharge rates from wheat and maize are lowest. The trend of variation in seasonal recharge rates is similar for both cotton and wheat. The lowest recharge rates are for S-SL due to lower gross irrigation requirements but increased under M-SL and D-SCL.
Seasonal recharge rates from vegetables are 606, 626 and 657 mm for S-SL, M-SL and D-SCL, respectively. Due to low capillary rise contribution, the gross irrigation requirements for vegetables are higher, which caused high recharge rates. The monthly trend for vegetables does not vary much due to almost similar monthly net irrigation requirements.
Table 4.6 Monthly and seasonal recharge (mm) determined from the water balance model at field level for different soil and groundwater characteristics and different crop types
(a) Shallow GW-silt loam
Month Cotton Wheat Vegetables Perennials Maize Rice
A 0 132 0 126 0 0 M 0 161 0 109 0 0 J 119 184 188 119 0 349 J 333 0 193 123 109 363 A 409 0 225 198 184 365 S 0 0 0 203 189 365 Season 861 477 606 878 482 1442
(b) Medium GW-silt loam
Month Cotton Wheat Vegetables Perennials Maize Rice
A 0 123 0 123 0 0 M 0 193 0 147 0 0 J 160 186 198 160 0 349 J 330 0 203 119 105 363 A 372 0 225 161 147 365 S 0 0 0 210 196 365 Season 862 502 626 920 448 1442
(c) Deep GW-silt clay loam
Month Cotton Wheat Vegetables Perennials Maize Rice
A 0 139 0 139 0 0 M 0 203 0 181 0 0 J 139 195 209 139 0 381 J 360 0 223 149 135 397 A 396 0 225 186 172 379 S 0 0 0 203 189 369 Season 895 537 657 997 496 1526
equation (4.6), respectively, for the year 2007. Recharge rates are higher from June to August in all HRUs. July and August are the peak months of irrigation for cotton, which explains why recharge is so high during that period. Average recharge is 4 mm d-1, which represents 57 % of the average gross irrigation amount.
Table 4.7 Monthly averages of daily recharge rates (mm d-1) for different hydrological response units in the Shomakhulum WUA for the year 2007
April May June July August September
HRU1 1.7 1.7 5.0 5.9 7.4 1.7 HRU2 1.1 1.3 4.6 7.8 9.6 0.2 HRU3 1.2 1.3 4.6 7.8 9.6 0.4 HRU4 1.7 3.8 5.4 7.4 8.2 0.8 HRU5 2.0 3.0 5.7 4.4 5.0 1.6 HRU6 1.1 1.5 5.7 7.7 8.7 1.3 WUA 1.4 2.0 5.2 6.6 7.7 1.2
The characteristics of HRU-1, HRU-2 and HRU-3 are similar (S-SL), but the recharge rates of HRU-1 differ substantially from those of HRU-2 and HRU-3. Although the cropping patterns of the latter two do not differ much (Table 4.2) and the share of cotton is the same in both HRUs. The same share of cotton resulted in similar recharge rates during the peak irrigation season of cotton (June-August). Slightly higher recharge rates for September in HRU-3 are due to a higher share of perennial crops in this HRU. In contrast, HRU-1 has a less than 50 % share of cotton compared to HRU-2 and HRU-3, which resulted in the different recharge rates. The higher recharge rates for HRU-1 during the early season (April and May) and even at the end of season (September) are due to the larger share of perennial plants compared to HRU-2 and HRU-3.
The characteristics of HRU-5 and HRU-6 are similar (M-SL), but both have different recharge rates. In HRU-5, wheat is the dominant crop providing higher recharge rates during April and May, whereas HRU-6 has a higher share of cotton, providing higher recharge rates during July and August. However, it is clear that the recharge rates in HRU-5 for April and May are much lower than those in HRU-6. This is due to higher recharge rates for cotton at field level as compared to wheat.
The characteristics of the HRU-4 (D-SCL) are different from all other HRUs, and therefore recharge rates cannot be compared to those in the other HRUs. Cotton is a
dominant crop in this HRU (more than 50 % of the total cropped area), which leads to higher recharge rates from June to August.
Water users association level
After determining the recharge rates at HRU level, recharge was up-scaled from this level to the WUA level using equation (4.6) The monthly recharge rates at the WUA level show that the recharge rates in April, May, and September are 1.4, 2 and 1.2, respectively, which is quite low compared to June (5.2 mm d-1), July (6.6 mm d-1) and August 7.7 (mm d-1). The recharge rates increase almost linearly, and maximum is
during August, while they are reduced substantially during September (Figure 4.5). Monthly trends of recharge rates in most of the HRUs do not differ much, i.e., most show higher recharge rates from June to August due to the higher share of cotton in the WUA, and lower recharge rates in April, May and September due to low irrigation supplies against the low irrigation water demand (Table 4.3).
Figure 4. 5 Recharge rates determined from the water balance model at WUA level during the vegetation season 2007