Water footprint of paddy cultivation under controlled runoff conditions: A case study in Kurunagala district, Sri Lanka

Paddy cultivation is the largest global consumer of water, and it also significantly contributes to water pollution. Investigating the water footprint of paddy agriculture can provide insights into how pollutants affect the ecosystem. This study aimed to quantify the total water footprint for paddy grown in Sri Lanka's Low Country Intermediate Zone under supplementary irrigation. A lysimeter study was carried out to determine the amount of leached nutrients below the root zone. The experimental design was a Complete Randomized Block Design (CRBD) with two factors (cropping season and gradient) and two levels (Yala and Maha; upper and lower). The green and blue water footprints for both sites were estimated using the CROPWAT 8.0 model by crop water requirement option. The results revealed that the loss of NO3-N through leaching accounted for 8.61 ± 1.84 kg/ha (8%), and the leaching losses of PO43-- P were 0.49 ± 0.1 kg/ha (2%) under controlled runoff conditions during the experimental period. The nitrogen fertilizer-induced grey water footprint (WFgrey) for one tonne of rice produced was 193 ± 27 m3/t, and the phosphorous fertilizer-induced WFgrey was 61 ± 7 m3/t. The study identified nitrate as the critical element for water pollution. The estimated total water footprint (WFtotal), which was the sum of green, blue, and grey water footprint, was 1409 ± 95 m3/t under controlled runoff conditions, while the global average value is 1325 m3/t. The estimated value is about 6% higher than the global average value. Therefore, these findings demonstrate the need for further research.