Estimation of Irrigation Requirement and Schedule in Southern Odisha for Major Rabi Cereal Crops using FAO CROPWAT 8.0 Model

Agricultural sector is facing two major challenges: the ever-increasing population growth that places continuous increase in the demand (Hedayat, 2005) and the need to improve utilization methods of the limited water resources (Falkenmark and Rockstrom, 2004). India faces high water stress and the country is amongst those with the most fragile and uncertain water resources in the world. Further, the agriculture uses 78 per cent share of water use in in the country (Mancosu et al., 2015). One of the main responses to these emerging challenges is to focus on improving water productivity in agriculture, as even small improvements will have large implications for local and national water budgets and allocation policies (Global Water Partnership 2000).
       
During rabi (Rain free period), the crops like maize, bajra and sorghum are cultivated under irrigated conditions in India. There is an increase in cultivated area during rabiseason which has a significant impact on food grain production in the country through increased cropping Intensity (Waha et al., 2020).Selection of right crop and scheduling irrigation enhances the water productivity. For proper use of limited water, there is a need to know the water requirement of crops for selection of them according to water availability and its proper scheduling. Such information is not available for southern Odisha.In absence of estimations of water requirement of crops through experimentation, the model-based approach serves as decision support system for optimal scheduling of irrigation thus paving a way towards increased water productivity.The  FAO CROPWAT 8.0 is a model based approach developed by the Department of Land and Water Resources, FAO for the calculation of crop water demand based on soil, climate and crop data. This is one of the precise and globally accepted models that serves as a decision support system by estimating the crop evapo-transpiration and effective rainfall thus computing the net irrigation requirement of the crop. Further, this software also suggests the dates for scheduling irrigation targeting maximum water productivity (Surendran et al., 2015).
       
Majority of rural people in southern Odisha earn their livelihood fromagriculture. This region is conducive for cultivation in all the three seasons if moisture is not limited. The cereal production in rabiseason of south Odisha region is predominated by maize, sorghum and pearl millet.
              
Keeping this in view, the crop water requirement and irrigation schedule for rabi maize, sorghum and pearl millet cultivated in red sandy loam soils of Southern Odisha was estimated using FAO CROPWAT 8.0 and presented in this paper.

Study area
 
The study area selected for the estimation of irrigation water requirement was Southern Odisha located between longitude 72°97¢ to 85°05¢E and latitude 18°34¢ to 20°66¢ N. The meteorological data of Koraput was used which is located between longitude 81°24.2¢ E to 84.2° E  and latitude 17°4¢ N to 20°7¢ N. This area is mainly dominated by red and lateritic soils.
 
Methodology
 
The CROPWAT 8.0 model developed by the Department of Land and Water Resources of FAOis a computer program for the calculation of crop water demand based on soil, climate and crop data. In addition, the program allows the development of irrigation schedules for different management conditions and the calculation of scheme water supply for varying crop patterns (Roja et al., 2020a). CROPWAT 8.0 can also be used to evaluate farmers irrigation practices and to estimate crop performance under both rainfed and irrigated condition. The basic functions of this model include estimation of reference evapo-transpiration, crop evapo-transpiration and irrigation water requirement.
 
Data used for estimation of ETo
 
Monthly climatic data viz, minimum temperature, maximum temperature, relative humidity, wind velocity, duration of bright sunshine hours of Koraput located in studyarea was used (Table 1) for the estimation of reference evapo-transpiration (ETo) using Penman-Monteith formula by FAO CROPWAT 8.0.


 
Rainfall data
 
Monthly rainfall data of Koraputwas used for calculation of effective rainfall (Table 1). USDA soil conservation method is used in this software .
USDA Soil Conservation Service formula:
Case 1: Peff = Pmon * (125 - 0.2 * Pmon) / 125  
              (If  Pmon<= 250 mm)
Case 2: Peff = 125 + 0.1 * Pmon (If Pmon > 250 mm)
Peff = Effective Precipitation
Pmon = Monthly Precipitation
 
Crop data
 
In Southern Odisha, during rabi season, the cultivation of crops usually begins after the harvest of kharif rice. In general, rabi maize, sorghum and pearl millet are sown around 15th of November and theses crops - maize, sorghum and pearl millet take 120, 110 and 100 days respectively to complete their life cycle. Duration and sowing date mentioned in the software helps to determine the phenophase of the crops. At each phenophase Kc value, rooting depth (m), critical depletion value and crop yield response factor were decided based on the duration of the crops (Doorenbos and Kassam, 1979) and presented in the Table 2. The yield response factor (Ky) is the ratio of relative yield reduction to relative evapo-transpiration deficit that integrates the weather, crop and soil conditions that make crop yield less than its potential yield in the face of deficit evapo-transpiration also estimated in the model. The crop data used in this experiment were presented in Table 3.




 
Soil data
 
The soils in Koraputare mainly of two types 1) Alfisols, 2) Ultisols (DLIC, 2016). In the present study, red sandy loam soils were considered based on their predominance in this region. The total available soil moisture, maximum infiltration rate, maximum rooting depth, initial soil moisture depletion of this soil type were in the tune of 100 mm/meter, 30 mm/day, 900 cm, 50 per cent, respectively and the initial soil moisture was 50 mm/meter which is 50% of available soil moisture.
 
Crop water requirement
 
The crop water requirement for the given crop duration was computed using the input viz. climate, crop and soil data of the region. The crop evapo-transpiration (ETc) was calculated by the equation:
Where,
Kc is the crop coefficient, ETc is crop evapo-transpiration and ETo is reference evapo-transpiration. Difference between crop evapo-transpiration and the effective rainfall determines the total irrigation requirement of the crop.
 
Irrigation scheduling
 
Irrigation scheduling determines when to irrigate and how much amount of water to be given to the crop at each irrigation. In this study, irrigation was scheduled at 50 per cent critical soil moisture depletion and irrigation was applied till the soil is refilled to field capacity at 70 per cent efficiency.

Estimated reference evapo-transpiration
 
The reference evapo-transpiration was maximum in the month of May (5.45 mm/day) and least was estimated in the month of December (2.34 mm/day) (Table 1). This might due to high average temperature, wind velocity, sunshine hours in the month of May resulting in increased atmospheric demand for moisture and vice versa in the month of January (Luo et al., 2021; Todorovic et al., 2013).
 
Crop water requirement
 
The value of daily or decadal crop water requirements for rabi maize, sorghum and pearlmillet for Southern Odisha was presented in the Table 4. The highest crop water requirement in maize occurs in the midseason phase, during third decade of January (32.3 mm). While the lowest water demand value was observed during initial phase. This might be due to less metabolic activities in the plants during initial days of crop growth while the plants need the most water during cob formation phase (Roja et al., 2020b; Roja et al., 2020c). The crop water requirement estimated by CROPWAT 8.0 for the rabi maize in Southern Odisha was 252.3mm. Similarly, for sorghum and pearl millet, the total crop water requirement estimated to be 197.9 and 172 mm, respectively. The irrigation requirement for rabi maize, soghum and pearl millet was computed after estimating the actual rainfall contribution in satisfying the part of water requirement. The computed irrigation requirements for rabi maize, sorghum and pearlmillet  were 231.2, 178.8 and 156.3 mm, respectively.


 
Irrigation scheduling
 
In Southern Odisha, under red sandy loam soils the  irrigation scheduled at 50 per cent of critical depletion with a quantity of water sufficient to refill the soil to 100% field capacity, the maize, sorghum and pearl millet require 11, 7 and 7 irrigations respectively  during their  and growing periods. The net irrigation requirement of maize, sorghum and pearlmillet were 280, 202.2 and 182.2 mm respectively and gross irrigation requirement was 399.9, 288.8  and 260.2 mm, respectively. Similar findings were reported by Ramulu et al., (2010) in maize; Kumar et al., (2021) in pearlmillet; Rahma et al., (2018) in sorghum. The detailed irrigation scheduling is reported in Table 5 and Table 6.

Due to inclusion of wide range of inputs for running of FAO CROPWAT 8.0 viz. meteorological, soil and crop data from the study area made it a most ideal decision-making tool in irrigation planning. In the present study, it can be concluded that rabimaize requires maximum water requirement followed by sorghum and pearl millet. Maize requires eleven irrigations followed by seven irrigations by both sorghum and pearl millet respectively.Accepting this model-basedapproach in scheduling irrigation is expected to have wide scope in reducing the wastage of irrigation water and to improve water productivity of a crop in a given region.

None.