Computational configuration of an agricultural irrigation rainwater harvesting system in karst regions based on drought characteristics

Rainwater harvesting systems are key measures for collecting rainwater for irrigation in rain-fed agricultural regions. As global drought events increase, aligning the construction of rainwater harvesting systems with irrigation demands under regional drought conditions has become a research priority in agricultural irrigation. In Guizhou Province, located in the core karst area of southern China, the frequent alternation between rainfall and drought makes effective rainwater collection for drought mitigation a critical support for regional agricultural development. To address this, this paper derives a method for calculating the configuration of rainwater harvesting systems based on regional drought characteristics and irrigation demands. Analysis of 51 years of climate data (1970–2020) revealed that: (1) Regional rainfall is mainly concentrated from April to September, accounting for 70% of the annual total, while ET C is concentrated from March to August, accounting for 60% of the annual total. During January–March, mid-July to mid-August, and December, ETc exceeds the rainfall during the same period, indicating water deficits and highlighting the need for rainwater storage to enable temporal redistribution. (2) Regional drought events are frequent, with short-term droughts at 1-month and 3-month scales accounting for 60.54% of events. These occur mainly in winter–spring (December–March) and during the summer–autumn transition (August–September), making them the dominant drought types in the study area. Based on these findings, this paper derives calculation methods for determining catchment area, high-level storage reservoirs volume, and low-level collection reservoirs volume in rainwater harvesting systems suitable for this region, and provides core algorithm parameters such as rainy season duration, rainfall amount, and drought duration. Field validation at three sites showed that rainwater harvesting systems configured using this method achieve a collection efficiency of 77%, with no significant differences among sites. This provides a viable technical solution for rainwater harvesting and drought resistance in agricultural production in karst regions.