Human activities and extreme hydrological events are posing continuing threats to sustainable management of water resources in Gin catchment, Sri Lanka (972 km²), a rain-fed agriculture-dominated catchment in the tropics which experiences uneven temporal rainfall distribution. This study aims to assess the seasonal water availability in the Gin catchment by setting up of Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) rainfall–runoff model. The HEC-HMS continuous-based simulation performed well with Nash–Sutcliffe efficiency, NSE ≥ 0.72 and percent bias, PBIAS ≤ − 6.8% during the calibration (2011–2014) and validation (2015–2017). The seasonal and annual water balance volumes during 2011–2017 were simulated reasonably well with 10–13% error and 3% error on average, respectively. The modeling results indicated clear seasonal variation with respect to the availability of water depicting 64% of the annual water yield generation in the cultivation seasons, Yala (April–June) and Maha (October–December), dominated by the monsoon rains. The direct surface runoff regime has become the dominant component in the two cultivation seasons (68–70%), and the precipitation loss volume constituted higher percentage in the non-cultivation season (44%) in comparison with the cultivation seasons (28%). Even during the non-cultivation season which coincides with the dry months of the year (January–March and July–September), a significant contribution of baseflow to the total streamflow (40%) was evident which will favorably impact the rain-fed perennial crops. Maintaining the environmental flow requirement, Q₉₀ (≈ 18 m³/s), in the river during the driest month (March) should be given attention in the proposed Gin–Nilwala inter-basin diversion plan in the upstream catchment.

人类活动和极端水文事件对斯里兰卡 Gin 流域（972 km ^{2）水资源的可持续管理构成持续威胁。}），热带地区以雨水灌溉农业为主的流域，其降雨时间分布不均匀。本研究旨在通过建立水文工程中心-水文模拟系统（HEC-HMS）降雨径流模型来评估Gin流域的季节性可用水量。在校准（2011-2014 年）和验证（2015-2017 年）期间，HEC-HMS 连续模拟表现良好，纳什-萨克利夫效率 (NSE ≥ 0.72) 和偏倚百分比 (PBIAS ≤ − 6.8%)。2011-2017年季节和年度水平衡量模拟得相当好，平均误差分别为10-13%和3%。模拟结果表明，可用水量存在明显的季节性变化，占雅拉耕作季节年产水量的 64%（4 月至 6 月）和玛哈（10 月至 12 月），以季风降雨为主。直接地表径流在两个耕作季节（68%~70%）成为主导，降水流失量在非耕作季节（44%）所占比例高于耕作季节（28%） 。即使在一年中干旱月份（一月至三月和七月至九月）的非耕作季节，基流对总水流的显着贡献（40%）也是显而易见的，这将对雨养多年生植物产生有利的影响农作物。保持环境流量要求，Q ₉₀ (≈ 18 m ³/s），在建议的上游流域的金-尼尔瓦拉跨流域引水计划中应注意最干旱月份（3月）河流中的水污染。