Combined effects of temperature and precipitation on the spring runoff generation process in a seasonal freezing agricultural watershed

This study aims to investigate the combined effects of temperature and precipitation on the hydrological processes in a watershed with intensive agricultural land uses during the spring snowmelt period. Temperature, precipitation, soil moisture, frozen soil depth, and discharge were monitored during the freezing and thawing periods in 2014, 2015, and 2016 within the 75 km2 agricultural Heidingzi watershed in northeast China. The results indicated that high autumn rainfall and early precipitation, as well as the corresponding temperature rise, during the stable freezing period, increased the amount of surface water stored in the ice form before the spring runoff. These conditions produced a prolonged and increasing runoff event, and the highest runoff ratio of the ice melt runoff process during the thawing period in 2014. However, low autumn rainfall, dispersed precipitation, and negligible temperature rises during the stable freezing period in 2014–2015 led to a significant but short-term snowmelt runoff during the thawing period in 2015. Because of similar precipitation conditions as in 2014–2015 and a temperature rise event during the freezing period in 2015–2016, the runoff during the thawing period in 2016 was a combination of snow and ice melt; the runoff ratio during the early- and late-melt stages in the maize-dominated drainage region (DR) was the highest with the lowest precipitation. Additionally, the early low rainfall during the thawing period in 2014 increased the direct runoff ratios by 2–13 times for the entire watershed, DRM, and DRP-M, as early rainfall resulted in small soil thaw depth and low water storage. Different land use activities in the agricultural watershed supported the spatial and temporal differences and uncertainties in the spring snow or ice melt runoff generation process. Moreover, snowmelt simulation models can rarely distinguish between ice and snowmelt during spring runoff generation processes, probably leading to high uncertainty in simulating spring runoff response to climate change in seasonal freezing areas. This study reveals the characteristics and causes of snow and ice melt runoff processes in agricultural watersheds that experience seasonal freezing and provides a new perspective for improving the modeling of water generation processes.