Effects of rainfall process on runoff in small watersheds in the Loess Plateau of western Shanxi Province, northern China

ObjectiveThis study aims to discuss the effects of precipitation process (rainfall patterns) on runoff in small watersheds, and provide the basis for the analysis of runoff and confluence on a small watershed scale.

MethodCaijiachuan Small Watershed of the National Field Research Station of Forest Ecosystem in Jixian County, Shanxi Province of northern China was taken as the research object. Rainfall and runoff data were collected from 2007 to 2017 in the Caijiachuan Watershed and rainfall patterns were divided according to the location of rainfall in the precipitation process. LSD multiple comparison method was used to study rainfall characteristics of different rainfall patterns on the basis of rainfall process. The response relationship between rainfall pattern characteristics and runoff was explored in small watershed scales of different land use types.

Result(1)The storms were divided into four rainfall patterns: rainfall concentrated early in the rainfall (Type Ⅰ); rainfall concentrated in the middle of the rainfall (Type Ⅱ); rainfall concentrated at the end of the rainfall (Type Ⅲ); and rainfall distributed uniformly over time (Type Ⅳ). Type I was characterized by shorter durations and higher intensities, which was the main type of rainfall, accounting for 44.6% of the total rainfall frequency. The average rainfall was 24.6 mm and rainfall erosivity was the strongest, which concentrated in June to August. Type Ⅳ had longer durations and lower intensities, with an average duration of 716 minutes and an average rainfall of 17.84 mm, rainfall erosivity was the weakest, mostly occurring in the early (May) and late (September) phases of the rainy season. The characteristics of Type Ⅱ and Type Ⅲ were between TypeⅠ and Type Ⅳ, which mostly occurred from June to August. The distribution of rainfall and frequency was relatively balanced. (2) The order of runoff depth and peak discharge in small watersheds under different rainfall patterns was Type Ⅰ > Type Ⅲ > Type Ⅱ > Type Ⅳ, lag time of flood peak was Type IV > Type II > Type III > Type I. (3) Runoff depth and peak discharge in the agricultural small watershed under Type I and Type III were higher than that in the closed small watershed, which was the opposite under Type II and Type IV. The peak flow appeared earlier in the agricultural small watershed. (4) Rainfall and duration had a greater impact on the runoff in the agricultural small watershed, intensity distribution had greater impact on the runoff in the closed small watershed.

ConclusionThe rainfall characteristics of different rain patterns are significantly different. Type Ⅰ was the main rainfall pattern causing the runoff of small watersheds. The closed small watershed had a more significant role in reducing peak discharge and prolonging the appearance time of peak flow than the agricultural small watershed, especially for heavy rainfall and strong rainfall, the regulation effect was more obvious. The rainfall, rainfall duration and rainfall intensity distribution in the Loess Plateau of Western Shanxi Province are the important factors influencing the runoff production in small watershed.