Rainfall Probability Analysis, Runoff Assessment and Water-Harvesting-Based Cropping Planning in Vihokhu Watershed, Dimapur, Nagaland, North-Eastern India

Abstract

Aims: The study was undertaken to assess dependable rainfall availability, estimate runoff and crop-water surplus or deficit, and develop a water-harvesting-based cropping plan for Vihokhu watershed of Nagaland, India. Study Design: Hydrological data for a 15-year period (2007–2021) were analysed for annual, monthly and weekly rainfall, dependable rainfall probability, runoff generation, crop-water requirement and water-storage potential. Place and Duration of Study: Vihokhu watershed, Kuhuboto Block, Dimapur district, Nagaland, India; rainfall data covering 2007–2021 were analysed. Methodology: The watershed covers approximately 807 ha. Rainfall data obtained from the Dimapur Soil and Water Conservation Office were analysed at annual, monthly and weekly intervals. Weekly rainfall was estimated at 60, 70, 75 and 80% probability levels using probability analysis, with the 75% probability level selected for agricultural planning. Runoff was estimated using Binnie's runoff coefficient method. Crop-water requirements were assessed weekly for crops grown or proposed for Kharif, Rabi and Zaid seasons. Surplus runoff was evaluated for storage and subsequent irrigation use. Results: The watershed received an average annual rainfall of 1564.11 mm during 2007–2021. Annual rainfall ranged from 1047.8 mm in 2017 to 1876.7 mm in 2012. July was the wettest month, receiving an average of 312.34 mm, while January and December received only 17.26 and 16.98 mm, respectively. At 75% probability, rainfall was absent during Weeks 1–14 and 41–52, indicating a prolonged dry period. Estimated runoff was concentrated mainly during Weeks 15–40. The accumulated runoff available for storage was estimated at 233.54 ha-m, of which approximately 151.80 ha-m was considered effective after accounting for 35% losses. The proposed cropping plan covered 175 ha and included rice and soybean in Kharif, mustard, beans, pea and onion in Rabi, and maize and potato in Zaid. Three water-harvesting ponds were proposed for storing runoff for non-monsoon irrigation. Conclusion: Vihokhu watershed has considerable potential for integrated rainfall-runoff management. Harvesting monsoon runoff and storing it for dry-season irrigation can support multi-season cropping and improve utilisation of available water resources.