Calibration of Crop Coefficients and Evapotranspiration Rates in Semi-Arid and Sub-Humid Agro climates: Impact on Crop Water Requirement
Abstract
Crop water requirement, a key component for Irrigation planning and management depends on Actual Evapotranspiration rates. Variations in Evapotranspiration rates depends on the climatic conditions for a given soil and crop. The objective of this work is to determine the water consumptive use based on crop coefficients for Tomato in Semiarid and Sub-humid agro climates. The Actual Evapotranspiration was quantified by Lysimeters. Sieve analysis of the soil indicated as sandy soil and has density of 1.859*10-3 Kg/cm3 . Depending on density and the root depth of tomato crop, lysimeter of dimensions 52cm depth and 36cm diameter is used to measure actual evapotranspiration rate.
Regression analysis carried out for the actual evapotranspiraton rates, computed using empirical formula indicated that the FAO-56 PM method is well suited for both the regions having correlation coefficients of 0.94 and 0.92 for Semiarid and subhumid regions respectively. Further, it was found that Thornthwaite equation being the next suited method has a correlation of 0.90 for semiarid and Hargreaves method the next suited method with correlation of about 0.90 for sub-humid. Crop coefficients used in all this potential Evapotranspiration methods were calibrated with lysimeter insitu measurements.
The crop coefficients vary depending on the different crop stages. The recalibrated crop coefficients for tomato crop are 0.78, 1.045, 1.95 and 1.54 for initial, development, mid and late respectively for Semi-arid Agro-climate. Similarly for SubHumid agro climate the crop coefficients were found to be 0.9, 0.98, 1.55 and 1.3 respectively.
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Introduction
Fresh water is a finite resource, is limited in summer and the demand for Agriculture is continuously increasing in Asian countries (Ravikumar et al., 2011). Water availability for agriculture depends on climate and the different losses in the existing water cycle. It is well known that water is a major issue almost in all parts of the world especially for countries that have high population growth rates and thus more crop yield (Dinar et al., 2019). Water deficit owing to the temporal-spatial inconsistency between water supply and demand is expected to become harsh in present scenario. Recently, climate changes has shown imbalance in the losses like evapotranspiration rates, thus varying delta for the crop growth (GoI 2016; Surendran, 2019). Development of irrigation systems with efficient use of water is essential for the sustainability of the crop production system and accurate estimation of crop water use (evapotranspiration) is also a critical component for water resource management (Petropoulos et al., 2018). Evapotranspiration rates, one of the key components required for rainfall runoff modeling, reservoir management and other integrated approaches in agriculture dominating watersheds (Danlu et al., 2016). A better understanding of trends in potential evapotranspiration (ET0) is crucial for scientific management of water resources in varied Agro climates (Dinpashoh et al., 2018).
Accurate estimations on crop water requirement are needed to avoid the excess or deficit water application, with consequent impacts on nutrient availability for plants, soil salinity and groundwater contamination. Evapotranspiration (ET) is an important component in water-balance models and irrigation scheduling, and is often estimated in a two-step process. The evaporative demand of the environment is estimated based on weather conditions, and is often estimated as the evapotranspiration from a theoretical, reference grass crop (ETo) with the crop defined as an actively growing, uniform surface of grass, completely shading the ground, and not short of water. The ETo value is then adjusted to estimate the evapotranspiration of the particular crop of interest using a crop-specific crop coefficient (Fisher and Pringle, 2013)
Conclusion
Irrigation requirements are developed with the help of crop water requirement of every crop. Hence for designing the irrigation system Evaluation of ET rates are very essential. Actual Evapotranspiration using various Empirical methods and field experiments by lysimeters and recalibrating the crop coefficients for different agro climates and upcoming with the suitable method for computation of Evapotranspiration rates for the respective climates. The average Actual ET for tomato crop by lysimeter method for Semi-Arid region was estimated to be 8.39 mm/day and in Sub-Humid region it was about 7.77 mm/day and therefore the corresponding correlations with FAO-56 PM equation was found to be 0.94 for Semi-Arid region and 0.92 for Sub-Humid region. Thornthwaite equation when correlated with the lysimeter gave a value of 0.9 and therefore is next well suited method for computing ET in semi arid region. In the same way Hargreaves Samani Equation correlated well with a value of about 0.9 and hence is the next well suited method in Sub Humid region. Due to large climatological data requirement by FAO-56 PM equation it becomes non feasible for estimation ET though it gives accurate results, But in some areas climate data are not available, there this method is difficult to carry out. Together with weather forecasts of rain and evapotranspiration for the next days, such a simulation model platform could be part of a decision support systemon crop irrigation. In a next step, the simulated Kc curves could be generated and mapped for other field crops and for relevant agronomic regions including applications with national and global data sets with noticeably differing weather conditions including gridded weather data.