Estimating crop water requirements for irrigation scheduling in different crops in humid subtropical agro-climate of Western Himalayas

Deficiency of water in the rhizospheric area of soil results in reduced crop growth adversely affecting crop yield. Thus, the objective of irrigation is to maintain the adequate moisture content in the root zone, such that the crop yield is not adversely affected (Kumar et al. 2012; Kumar et al. 2020). However, in the current scenario of climate change, rapid industrialization and population increase, there is tremendous pressure on water resources both quantitatively and qualitatively (Rao and Poonia 2011). Hence, precise allocation of water resources considering the crop water requirement and proper knowledge of soil composition is essential to attain optimum yield, and maximum water use efficiency (Mehta and Pandey 2016). The crop water requirement is generally crop evapotranspiration (ETc) considering atmospheric water loss through plant transpiration and soil evaporation simultaneously (Kumar 2017; Poddar et al. 2021). There are several methods for direct estimation of ETc which includes energy balance, microclimatological methods, field water balance and Lysimeters. However, the indirect methods includes measurement of reference evapotranspiration and crop coefficients (Chaudhari et al. 1999). Estimating water balance in the Lysimeter is the most reliable and useful approach for determining actual ETc under field conditions (Kashyap and Panda, 2001). Hence, the present study was undertaken for rabi (Wheat, Indian mustard, Potato) and kharif (Maize, Sorghum, Guar) crops grown in a humid subtropical agro-climate of western Himalayas with a specific objective to estimate the ETc using the lysimeter water balance approach. Description of the field experiments

field trials were conducted during the 2017 -2019 crop growing seasons. The crops considered in the experimental study were wheat (Triticum aestivum), Indian mustard (Brassica Juncea), potato (Solanum tuberosum L.), maize (Zea mays), sorghum (Sorghum bicolor L. Moench) and guar (Cyamopsistetragonoloba L.). The details of the crop type, crop duration, growth stages, and irrigation events for the present study are illustrated in Table  1. Local agronomic practices (i.e., land preparation, manuring, fertilization) were followed while conducting experiments on different crops.

Water balance method
Two drainage type lysimeters (1.5×1.5×2 m) were installed separately and rim of the lysimeter was kept 0.1 m above ground level to prevent surface runoff. A land gravel filter (0.3m) was provided at the bottom of the lysimeter to facilitate drainage to a calibrated collector. A soil moisture capacitance probe was used to measure soil moisture content at 0.1 m interval till 1.6 m in the soil profile on daily basis. The applied irrigation was measured using a discharge meter installed at the outlet of the water tank. The daily rainfall was recorded using a tipping bucket rainfall.
The water balance method assesses the incoming and outgoing water flux into the crop root zone. Since drainage type lysimeters were used, ET c was obtained for long periods. The ET c was determined using the water balance equation (Bandyopadhyay and Mallick, 2003) as: where, P = precipitation (mm), I = irrigation (mm), D = Drainage from lysimeter (mm), RO = runoff (mm), and ΔS = change in soil moisture storage (mm). ΔS for a specific depth (d z ) for a specific time period is computed as: where θ z, final and θ z, initial is the final and initial moisture content, respectively in the soil profile in a discrete-time interval.

ET c variation in rabi crops
The seasonal ET c of rabi crops, i.e., Indian mustard, wheat, and potato, was 165.8 mm, 242.7 mm, and 308.7 mm, respectively ( Table 2). The precipitation received during the crop period of wheat, Indian mustard, and potato was 112.4 mm, 114.5 mm, and 108.5 mm, respectively. The amount of irrigation required for wheat, Indian mustard, and potato, was 195.0, 140.0, and 240.0 mm, respectively. This indicated that the irrigation requirements of rabi crops are higher than the precipitation received during the crop period. In the case of wheat and potato, the irrigation supplied was almost twice the precipitation, indicating that nearly twothirds of the crop water requirements were fulfilled by irrigation. The contribution to the groundwater during the rabi crop season was quite low as compared to the amount of water received. The variation of stage-wise ET c for rabi crops considered has been shown in Table. 2. It is evident from the table that the ET c during the initial and midseason stage for rabi crops was quite similar. However, during the crop development stage, the ET c for potato was significantly higher than wheat and Indian mustard. During the late season, the ET C for wheat was higher as compared to the other two crops.

ET c variation in kharif crops
The seasonal ET c of kharif crops, i.e., maize, sorghum, and guar, was 502.9 mm, 518.9 mm, and 494.7 mm, respectively ( Table 2). The amount of precipitation obtained during the crop period of maize, sorghum, and gaur, was 1052.5, 1059.3, and 1124.3 mm, respectively, which is comparatively much higher than the rabi crops. The total amount of irrigation required for maize, sorghum, and gaur, were 10.0, 31.4, and 20.0 mm, respectively, which is substantially less than the irrigation required for rabi crops. This also indicates that the irrigation requirements of kharif crops were very low as compared to the received precipitation. More than 95 per cent of the crop water requirements were fulfilled by the precipitation alone. The groundwater contribution during the kharif crop season was substantially high. The variation of stage-wise ET c for kharif crops considered has been shown in Table 2. The ET c for kharif crops during the initial and late-season stages was nearly the same. The ET c for sorghum was significantly low during the crop The seasonal ET c in the case of rabi crops was highly variable, but for kharif crops, it was nearly similar. The stage-wise ET c variation was different for each rabi crop considered. For kharif crops, maize and guar followed similar stage-wise ET c , whereas sorghum followed a different pattern. The estimated ET c of different crops may further help in planning of optimal irrigation schedules in the humid sub-tropical agro-climate of the western Himalayas.