Crop water requirement and irrigation schedule of paddy in South Gujarat Region using the CROPWAT 8.0 model

Introduction

Water is an essential resource for human survival and agricultural production, with agriculture accounting for nearly 70 per cent of global freshwater withdrawals. Growing population, climate variability and increasing competition for water among different sectors have intensified the need for efficient irrigation and water management. In this context, estimation of Crop Water Requirement (CWR) is crucial for determining the amount of water needed to meet crop evapotranspiration demands under optimum growth conditions (Allen et al, 1998; Kampman, 2007) [3,8] . Accurate assessment of CWR helps in planning irrigation schedules, improving water-use efficiency and ensuring sustainable crop production. Proper irrigation scheduling enables water to be applied in the right quantity and at the appropriate time, thereby avoiding both water deficit and excess water application. Consequently, it contributes to higher crop productivity, conservation of freshwater resources and maintenance of soil health. Effective Rainfall (P eff ) represents the portion of total rainfall that is effectively utilised by the crop. Irrigation Requirement (IR) is the amount of water that must be supplied through irrigation to meet the crop water demand after accounting for P eff . Food and Agriculture Organisation (FAO) has developed CROPWAT model for determining the CWR and irrigation scheduling of different crops which has been widely used across the globe for various applications (Boualem, 2023; Lee and Dang, 2019; Thimmareddy et al., 2023) [7,10,14] . Paddy was selected for the present study because it is one of the most important staple food crops and among the most water-intensive crops cultivated in India. In South Gujarat, paddy occupies a major share of the kharif cropped area and plays a significant role in agricultural production and food security. As the crop requires large quantities of water throughout its growing period, accurate estimation of CWR is essential for efficient irrigation scheduling, improving water use efficiency and ensuring sustainable management of increasingly scarce water resources.

Materials and Methodology

Gujarat was chosen for the study because of its limited water availability relative to agricultural demand and its diverse climatic conditions. Estimation of CWR can support efficient irrigation planning and sustainable water resource management in the state, which was categorised into four regions, North Gujarat, Middle Gujarat, Saurashtra and South Gujarat. The study focused on the South Gujarat regions, which have the 2 nd highest area (2942.59 00'ha) under paddy cultivation (Anon, 2023), favourable climate condition and dominant kharif crop in this region. Representative weather stations Valsad and Navsari from selected region were chosen for the analysis based on highest cultivated area. (Table 1). The crop growth season of paddy considered for the estimation of CWR is presented in Table 2. The paddy www.extensionjournal.com crop growth period was divided into five stages: nursery, initial, crop development, mid-season and late-season. (Allen et al., 1998) [3] . [13] Data for the years 2012-13, 2013-14, 2022-23 and 2023-24 were collected from various sources based on their availability. The climatic parameters considered for the study included mean maximum temperature (°C), mean minimum temperature (°C), mean relative humidity (%), mean wind speed (km day⁻¹), mean sunshine duration (hours day⁻¹), mean solar radiation (MJ m⁻² day⁻¹) and rainfall (mm month⁻¹). Weather data for the selected regions were obtained from the respective agricultural university of Gujarat. For the South Gujarat region, monthly weather data were collected from the Argo-Meteorological Cell, Department of Agricultural Engineering, N. M. College of Agriculture, Navsari Agricultural University, Navsari. (Anon, 2023 b ). The collected climatic data were used to estimate the ET 0 . CWR varies across different crop growth stages; therefore, K c values for paddy were adopted according to the FAOrecommended guidelines (Table 3). Soil parameters, including total available soil moisture (mm m⁻¹), maximum rainfall infiltration rate (mm day⁻¹), maximum rooting depth (cm), initial soil moisture depletion (% of total available moisture) and initial available soil moisture (mm m⁻¹), were also considered following FAO guidelines. (Allen et al., 1998) [3] The CWR was estimated using CROPWAT 8.0 models.

a. Reference Evapotranspiration (ET0)

The concept of ET 0 was introduced by FAO. ET 0 was calculated on the basis of the FAO Penman -Monteith equation (Allen et al., 1994 [1,2,3,9,11,12,13] .

Where, ET 0 = Reference Crop Evapotranspiration (mm day -1 ) R n = Net Radiation at the Crop Surface (MJ m -2 day -1) G = Soil Heat Flux (MJ m -2 day -1 ) T = Average Air Temperature ( °c) U 2 = Wind Speed measured at 2 m height (m s -1 ) E s = Saturation Vapour Pressure (kpa) E a = Actual Vapour Pressure (kpa) (E s -E a ) = Vapour Pressure Deficit (kpa) ∆ = Slope of the Vapour Pressure Curve (kpa °c-1 ) Γ = Psychrometric Constant (kpa °c-1 )

b. Crop Water Requirement (CWR)

The CWR was calculated by accumulating the data on the ET c (mm/day) over the complete growing period.

𝐶𝑊𝑅 = 10 * 𝐸𝑇 𝑐

Here, the factor 10 was included to convert mm into m 3 /ha. The ET c was calculated as follows:

Where, K c is the Crop Coefficient ET 0 is the Reference Evapotranspiration in a state (mm/day).

c. Effective Rainfall (Peff)

The P eff (Effective Rainfall) was generated from P tot (Total Rainfall) by CROPWAT (FAO, 2023 g ). CROPWAT calculates with a simplified version of the USDA method. (Dastane, 1978; Kampman, 2007) [15,8] . 3 and4, respectively. The results show that the total CWR of paddy ranged from 442.9 to 471.2 mm in Valsad and 467.6 to 505.4 mm in Navsari, with the highest values, 487.9 mm and 506.5 mm, in Valsad and Navsari, respectively, recorded during 2012-13. The higher CWR can be attributed to increased evapotranspiration during the mid-growth stage and less effective rainfall during the late-season stage, which requires more irrigation. Furthermore, paddy exhibited a declining trend over the decade in both districts. Navsari district shows high CWR in paddy because of less rainfall contribution as compared to Valsad, which needs more additional water requirement. Among the different crop growth stages, the mid-season stage recorded the highest ET c values, ranging from 147.3 to 153.8 mm in Valsad and 154.4 to 167.5 mm in Navsari. This was mainly due to full canopy development and peak physiological activity, resulting in maximum transpiration and water demand, along with additional evaporation from standing water in paddy fields. In contrast, lower ET c values were observed during the nursery and initial stages (20.0 to 41.1 mm and 57.5 to 62.4 mm in Valsad; 18.3 to 21.0 mm and 63.6 to 101.7 mm in Navsari), attributed to limited canopy cover, lower leaf area and reduced crop transpiration during early growth stages. www.extensionjournal.com

Note:

The sum of stage-wise Crop Evapotranspiration (ETc) values represents the total CWR for the crop during the entire growing period and LP = Land Preparation. Note: The sum of stage-wise Crop Evapotranspiration (ETc) values represents the total CWR for the crop during the entire growing period and LP = Land Preparation. www.extensionjournal.com

The P eff varied considerably across the study years, thereby influencing the IR of the crop. The total P eff ranged from 576.9 to 729.3 mm, while IR ranged from 198.5 to 206.1 mm in Valsad. In Navsari, P eff ranged from 561.3 to 715.0 mm, whereas IR varied from 112.7 to 237.4 mm. In both districts, supplemental irrigation was particularly required during the late growth stage to maintain adequate water levels in the paddy fields. Overall, in both districts, P eff is greater than IR, indicating paddy cultivation in south Gujarat depends on rainfall, there is no need for supplementary irrigation.

The crop water requirement (CWR) of paddy estimated in the present study was generally comparable with the findings of previous studies. Similar CWR ranges were reported by Kumar et al. (2024) [16] in different agro-climatic zones of Jharkhand (482.5-592.5 mm) and by Nath et al. (2022) [17] in Shivamogga district (542.4-606.1 mm). The CWR values observed in the selected districts of Valsad and Navsari were broadly consistent with these reported ranges, indicating the reliability of the estimated CWR under varying agro-climatic conditions. The irrigation dynamics of crop water use, efficiency and losses of paddy in Valsad and Navsari districts are presented in Tables 5 and6, respectively. The predominantly negative actual irrigation requirement across both districts indicates that P eff was generally sufficient to meet the water demand of paddy. Therefore, paddy cultivation in the study area was primarily rainfall-dependent, with irrigation serving a supplementary role. An irrigation efficiency of 100 per cent was assumed for both districts. High percolation losses were observed in both areas. However, substantial rainfall losses highlight the need for improved rainwater management practices.

Conclusion

The Crop water requirement and irrigation schedule of paddy were computed for Valsad and Navsari districts of South Gujarat using CROPWAT 8.0 model. The result revealed that in both district CWR of paddy varies among the different stages. In both district major depends on rainfall it means rainfall is sufficient to meet crop water needs, therefore supplement irrigation is less/not required to in both districts. Navsari District show higher CWR as compare to Valsad district. Higher gross and net irrigation requirement in Valsad district as compare to Navsari district. And both districts show negative actual irrigation requirement and high percolation losses and rainfall losses.

Acknowledgement

I would want to sincerely thank everyone who helped, advised, and inspired me throughout this research project. I also want to express my gratitude to the organizations and people whose help made this work possible.

References

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