Introduction

Water for agriculture is becoming increasingly scarce in the light of growing water demands from different sectors (IWMI, 2010). Analysis showed that the total crop water requirement of all major crops increased with the rising temperature thereby increasing the simulated irrigation water demand (Surendran et al., 2014). Crop water requirements (CWR) refer to the amount of water required to compensate the evapotranspiration losses from a cropped field during a specified period. Crop water requirements are expressed usually in mm day^-1, mm month^-1 or mm season^-1 and they are used for the management purposes: in the estimation of irrigation water requirements, irrigation scheduling and water delivery scheduling.

The primary objective of irrigation is to apply water to soil to meet crop evapotranspiration (ET_C) requirement when rainfall is insufficient to raise crops till harvesting. Cropwat is a FAO model for irrigation management designed by Smith (1991). It is meant as a practical tool to help agrometeorologists, agronomists and irrigation engineers to carry out estimation for evapotranspiration and crop water use studies, and more specifically the design and management of irrigation schemes. Recommendations for improved irrigation practices, the planning of irrigation schedules under varying water supply conditions, and the assessment of production under rain-fed conditions or deficit irrigation can be derived from this. Adequate data on irrigation water requirements of most crops is not available in developing nations of the world. This is one of the reasons why for the failure of large-scale irrigation projects in most developing countries of the world. The objective of this study was to determine crop water requirements of major crops like paddy and maize for the two seasons (kharif and rabi) in Bapatla channel command area using CROPWAT 8.0.

Materials and Methods

2.3.  Estimation of crop water requirement

The term crop water requirement is defined as the “amount of water required to compensate the evapotranspiration loss from the cropped field”. “Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration”. The crop ET (ET_C) was estimated by FAO Penman-Monteith equation:

ET_c= ET_o* K_c

Where, ET_c = actual evapotranspiration by the crop (mm day^-1),

K_c= crop coefficient at a specific growth stage

ET_o = reference evapotranspiration (mm day^-1) and it is determined by:

2.3.1.  Gross irrigation requirement (GIR)

The daily gross irrigation requirements for paddy and maize crops were computed using the following relationship:
where, NIR (Net irrigation requirement) = TWR–ER

=Crop water requirement + Special water needs – ER

ER=Effective rainfall, mm

USDA Soil Conservation method is used for calculating the effective rainfall (Kassam and Smith, 2001). In this method the effective rainfall can be calculated according to

ER=R(125-0.2*R)/125 for R<250 mm and                           

ER=125+0.1*R for R>250 mm                                          

Where, R=Rainfall, mm

η_a=Application efficiency (%)

In case of paddy, the special water needs include land preparation requirement, percolation losses and leaching. In the present study, percolation loss from paddy fields was assumed to be 2 mm day^-1. Paddy fields require about 10 days of land preparation. Water requirement for land preparation of paddy during these 10 days was taken as 200 mm. Since the percolation losses were already considered in computing water requirements of paddy, a higher value of 85% was used for the application efficiency in the case of paddy in comparison with 65% for the ID crops as recommended by Ministry of Irrigation, Govt. of India for arriving at gross irrigation requirements of paddy in the command area (Sarma and Rao, 1997).

Results and Discussion

3.1.  Reference evapotranspiration (ET_o)

Calculated average monthly climatic data and reference evapotranspiration of the study area were shown in Table 2. From Table 2, it was observed that the average peak monthly reference evapotranspiration (ET_o) was observed to be 6.65 mm day^-1 for the month of May. Whereas average minimum ET_o was observed as 3.28 mm day^-1 in the month of December and minimum temperature was observed in the month of January (18°C) and maximum temperature in the month of May (38.26°C).

Table 2: Average monthly climatic data and reference evapotranspiration of the study area

Monthly rainfall and effective rainfall (ER) of the study area for five years were calculated and tabulated in Table 3. Highest rainfall (1495.30 mm) and effective rainfall (1379.54 mm) were observed in the year 2013-14.

Table 3: Monthly rainfall and effective rainfall (ER) data of the study area for 5 years

3.2.  Crop coefficients

The crop coefficients (K_c) at different crop growth stages for paddy and maize crops were selected from Irrigation and Drainage Paper 56 (Allen et al, 1998) and adjusted for local climatic conditions as per the guidelines. The crop coefficients for paddy crop were found to be 1.05, 1.13 and 0.53 for the initial stage, mid-season and late season respectively and the crop coefficients for maize crop were found to be 0.15, 1.19 and 0.49 for the initial stage, mid-season and late season respectively. To determine consumptive use or crop evapotranspiration, K_c values are taken from the CROPWAT 8.0 for every 10 days of the crop season by substituting the above crop factors.

3.3.  Total water demand

For the Bapatla channel command area, total water demand was computed for the five years from 2012-13 to 2016-17 during two crop seasons of paddy crop (kharif and rabi) and maize crop for rabi season. Gross irrigation requirement of individual crops was calculated on the daily basis and then calculated monthly gross irrigation requirement.

For getting total water demand (TWD) for the entire year of the channel, gross irrigation requirement level of all crops was summed. Total water demand was calculated at monthly intervals and presented the calculations in Table 4 for five years. It was found that 6500.04 ha-m of water was required for irrigating entire command area in the year 2012-13. Similarly, for 2013-14, 2014-15, 2015-16 and 2016-17 years also, TWD was calculated and found to be 6325.18, 6521.42, 3102.40 and 5376.19 ha-m, respectively.

Table 4: Monthly total water demand for five years (2012-13 to 2016-17) in Bapatla channel command

Total water demand for the 5 years during kharif and rabi seasons was presented in the Figure 1. From the Figure 1, it was observed that total water demand (TWD) was least in the year 2015-16 (3102.40 ha-m) and maximum in the year 2014-15 (6521.42 ha-m). Net irrigation water requirement of Tarikere command area was reported to be 292.7 mm year^-1 (Nithya and Shivapur, 2016) whereas, gross water requirement (GWR) under Appapuram Channel Command was estimated to be 124.39 M.cum (Ratna Raju et al., 2016).

Figure 1: Total water demand for 5 years during kharif and rabi seasons in the Bapatla channel command area

Benin’s sub-basin of Niger River (BSBNR) annual reference evapotranspiration (ETo) was estimated at 1967 mm. The lowest monthly value of ETo of 123 mm, was observed in August month, middle of the rainy season while the highest value 210 mm was observed in March within dry season. The crop evapotranspiration (ET_c) and the crop irrigation requirements were estimated at 651 mm and 383 mm, respectively in rainy season and 920 mm and 1148 mm, respectively within a dry season (Bouraima et al., 2015). The crop water requirement for the groundnut Kharif and Rabi crops in the Anantapur region was estimated to be 591.3 mm and 443.3mm, respectively and for the vegetables, cotton, rice, grains and maize in the Anantapur region are to be 594.1 mm, 878.6 mm, 1110.6 mm, 699.9 mm and 679.3 mm, respectively (Babu et al., 2014).

Conclusion

Reference evapotranspiration, effective rainfall and crop water requirement can be estimated using CROPWAT with input of climatic data. The Reference evapotranspiration was estimated during the study period and found that the average peak monthly ET_o was observed to be 6.65 mm day^-1 for the month of May. Whereas average minimum ET_o was observed as 3.28 mm day^-1 in the month of December. Total water demand (TWD) of Bapatla channel command for two seasons (kharif and rabi) of crops paddy and maize was found to be 6500.04 ha-m in the year 2012-13. Similarly, for 2013-14, 2014-15, 2015-16 and 2016-17 years also, TWD was found to be 6325.18, 6521.42, 3102.40 and 5376.19 ha-m, respectively.

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