In dry region, water is limited and land is vast, hence, water management should aim at maximizing production per unit of water rather than per unit of land. Some of the technologies like extensive, deficit and pressurized irrigation have been developed to maximize the production in dry/arid regions. Pressurized irrigation has already been discussed in scheduling of irrigation headings.
Therefore, extensive and deficit irrigations approaches are discussed below:
1. Extensive Irrigation Approach:
Extensive irrigation approach seeks to apply a small quantity of water over a large area rather than large quantity of water over a small area. Production per unit land may decrease but production per unit water may increase in this approach. Wheat and mustard required 840 and 250 mm water per hectare, respectively to produce maximum yield.
When the same water was applied optimally in 3, 1.5 and 4.0 ha land in wheat and mustard, it gave less production per unit of land but total productivity per unit of water was more by bringing larger area under irrigation (Table 13.5). Singh (1997) observed that under given water supply, the area brought under irrigation in pearl millet was more in subnormal rainfall years than low rainfall years, however, the production enhanced in both the situation by bringing larger area under irrigation in pearl millet (Table 13.6).
2. Deficit Irrigation:
Water available for irrigation is scares in dry regions and a number of crops compete for finite source of water. Earlier emphasis was given on maximizing production per unit of land, which resulted in exploitation of groundwater leading to indiscriminate use of water. Deficit irrigation approach seeks to avoid irrigation at less critical stages of crop growth and apply less water at the end so as to eliminate water stored at harvest. One has to be well versed with the crop growth stages less sensitive to moisture stress and proper balancing between water given and ET demand of crop.
Therefore, field studies were carried out at many institutions in India to maximize the production per unit of water and to identify the optimum level of water deficit imposition in different stages and also to select the most water use efficient genotype of field crops under limited water supply conditions.
Work carried out on some of important dry regions crops are discussed below:
a. Wheat:
Singh and Mann (1979) studied in detail water requirement and sensitive growth stage of wheat to moisture deficit and deficit irrigation schedule. It was concluded that yield of 5400-5500 kg ha– 1 with ETm of 81 -83 cm was achievable in wheat cv Kalyansona with a k factor of 1.24- 1.33. The Y to ET relationship was linear.
However, Y to irrigation relationship varied in form from linearity under low range of irrigation to convexity under full irrigation applied to Ym. Thus, in arid region, irrigation in wheat below the required for Ym seems to have promise for rational use of limited water supply.
b. Groundnut:
The field studies were conducted on four genotypes of groundnut viz. GG2, TAG 24, GG 4 and GG 6 using single sprinkler line source design at Directorate of Groundnut Research, Junagadh, Gujarat during 2002-04. Six water gradients were imposed on three stages of crop growth viz. up to 30 days after emergence (DAE), 30-60 DAE and 60-90 DAE.
It was observed that-the most sensitive growth stage to water deficit was 60-90 DAE (pod formation to pod development) as value of B was the maximum whereas emergence to flowering (up to 30 DAE) was the less sensitive having B value the minimum. Among the cultivars, GG6 was the most sensitive and TAG 24 was the least sensitive to deficit irrigation (Table 13.7).
It was also revealed that cultivar TAG 24 had minimum evaporation of Etm (17%) compared to 22% in GG 6, showing more efficient user of water. The maximum yield under no limitation of water was 4650 kg ha -1 for TAG 24 and 3000 kg ha -1 for GG6 (Table 13.8; NRCG 2002-03 and 2003-04).
c. Rabi Crops:
In a study conducted at Bhuj, Gujarat, Devi Dayal et al. (2012) concluded that the least reduction in grain yield under deficit water supply of 40% of field capacity was recorded in mustard (11.6-19.1%) followed by fenugreek (9.5-20.0%) and barley (9.7-22.7%).
In Mustard, cv GM 2 showed higher reduction (19.1%) than that in cv GDM 4(11.6%). Fenugreek cultivar RMT 305 was superior to cv Guj 1 and had less reduction in grain yield (9.5%) than that in cv Guj 1 (20.0%). Barley cv RD 2715 showed higher reduction in grain yield (22.7%) compared with that of cv RD 2035 (14.1%) and cv RD 202794 (9.7%).
In wheat, Singh and Mann (1979) studied yield-water relationship and concluded that the most sensitive growth stage to deficit moisture supply was booting/heading period, but were relatively insensitive when wheat crop was conditioned to some 15% moisture stress in the vegetative stage. This led to the conclusion that if water is limited, the deficit should be spread nearly evenly over the previous growth stages. Preconditioning the crop to some stress seems to reduce the impact of water stress in subsequent stages of growth.
In groundnut, water deficit during flowering stage at 30-60 days after sowing (DAS) was found the most sensitive stage to moisture deficit causing pod yield reduction of 52% and water productivity of 2.65 kg ha-1 mm-1 as compared with no yield reduction and maximum water productivity of 5.08 kg ha-1 mm-1 under no moisture deficit. The least sensitive stage was early growth stage at 0-30 DAS.
In mustard, barley and fenugreek, the least and most sensitive stages to water deficit were identified as 90 DAS to maturity and early growth stage at 30-60 DAS, respectively. The corresponding value for yield reduction (%) and water productivity (kg/ha/ mm) for most sensitive stage were 35.03 and 3.67 for mustard, 35.82 and 6.36 for barley and 40.29 and 2.61 for fenugreek, respectively.
d. Intercropping System:
Deficit irrigation schedules were worked out for groundnut based intercropping system at Junagadh. Intercropping of groundnut with pearl millet and sesame in 1:1 and with castor and pigeon pea in 3:1 row ratio were considered under study. Six water gradients were provided using single line source sprinkler design. The system operated after the harvest of groundnut.
The results revealed that though irrigation at potential ET recorded the highest yield per unit area of land, WUE declined both in sole and intercropping systems under deficit irrigation schedule. Maximum WUE with added ET were 5.45 and 3.33 kg ha-1 mm -1 for pigeon pea and groundnut + pigeon pea intercropping, respectively, at 70% replenishment of water deficit of field capacity.
In case of castor based system, maximum WUE of castor (4.74 kg ha-1 mm-1) and castor + groundnut (3.12 kg ha-1 mm-1) was realized at 85% replenishment of water deficit of field capacity. But irrigation at 70% replenishment of water deficit of field capacity resulted in equally high WUE of 4.71 kg ha -1 mm -1 for sole castor and 3.11 kg ha-1 mm-1 for castor + groundnut. Production though per unit of land will decline, per unit of water will increase by bringing additional area under irrigation for a given water supply. Thus, deficit irrigation is an effective method of irrigation to increase yield per unit of water under dryland system.
e. Sequential Cropping System:
Yield and water productivity of prevailing cropping system in Indira Gandhi Nahar Pariyojna (IGNP) at stages I (Hanumangarh) and II (Bikaner) were assessed. Water applied through rainfall and irrigation was taken care for computing water productivity-economic yield (WP- EY) and water productivity-net returns (WP-NR).
It was revealed (Table 13.9) that cluster bean based cropping system in Hanumangarh region (Stage I) had higher WP both in physical and economic terms. In Bikaner region (Stage II), cluster bean-chickpea cropping system had higher WP-EY (0.42-0.63) and WP-NR (21.16-19.14) in both the years studied.
An attempt on simulation of WP in tested crops after calibration and validation of CropSyst Model was made by Yadav et al. (2016). In IGNP Stage I, it was concluded that combination of 200 mm irrigation and 150 kg N ha-1 is optimum for realizing maximum WP in Cotton. For rabi crops, 300 mm irrigation with 200 kg N ha-3 for wheat, 100 mm irrigation with 60 kg N ha-1 for mustard, and 400 mm irrigation with 150 kg N ha -1 for barley proved to be optimum for getting maximum WP. In IGNP Stage II, in cluster bean, 300 mm irrigation with 20 kg N ha-1 was economically better with respect to WP (0.29 kg m-3). In groundnut, increase in nitrogen level with 300 mm irrigation improved WP.
However, in wheat, increasing irrigation from 200 mm to 400 mm with increased level of N from 50 to 200 kg ha-1 was found better for higher WP. In mustard, irrigation of 100 mm with 100 kg N ha -1 gave maximum WP of 0.46 kg m-3.
The above results suggest that the CropSyst model can be used as an efficient tool for assessment and improving water productivity through water and nutrient management.