Effects Of Rainfall Regime On Soil And Water Loss At Loessial Hillslope With Ephemeral Gully

Understanding the effects of rainfall regime on soil and water loss at loessial hillslope with ephemeral gully was important to establish soil erosion prediction model and configurate conservation measures at hillslope. Based on 115 erosive rainfall events collected from runoff plots at Fuxian Soil Erosion observatory during 2003 to 2014, the study analyzed the characteristics of rainfall regime, quantified the effects of rainfall regime on soil and water loss characteristics at different erosion zones, exhibited effects of upslope runoff on erosion characteristics of downslope under different rainfall regimes, illustrated the impacts of rainfall regime on rill erosion and its morphological characteristics, and established equation between erosion amount and PI30 under different rainfall regimes and PI30 based on erosion patterns. The main results were as follows:(1) The characteristic of rainfall regime were analyzed. According to three rainfall parameters, precipitation, rainfall duration, and maximum 30 min rainfall intensity, 115 rainfall events were classified into three rainfall regimes by using K-mean clustering and Discriminant analysis. Rainfall Regime 1(RR1) had large maximum 30 min rainfall intensity with low precipitation and short duration, while the three rainfall parameters of Rainfall Regime 3(RR3) were inversed with those of RR1; for Rainfall Regime 2(RR2), precipitation, duration and maximum 30 min rainfall intensity were all between those of RR1 and RR3. RR1 had the largest I10, I15, I30, I60 and Im, followed by RR2 and RR3.(2) The effects of rainfall regime on soil and water loss at different erosion zones were illustrated. Runoff amount of sheet erosion zone caused by RR1, RR2 and RR3 occupied 67.5%, 21.4% and 11.1% of total runoff amount, respectively; while soil loss occupied 64.3%, 23.7% and 11.9% of total soil loss, respectively. For rill erosion zone, runoff amount and soil loss caused by RR1, RR2 and RR3 accounted for 60.4%, 24.1%, 15.5% and 70.5%,18.8%, 10.7% of total runoff amount and soil loss, respectively. For ephemeral gully erosion zone, runoff amount and soil loss caused by RR1, RR2 and RR3 accounted for 60.7%, 27.7%, 11.6% and 68.8%, 20.9%, 10.3% to total runoff amount and soil loss, respectively. For three erosion zones, runoff amount and soil loss of sheet erosion zone occupied 7.8%~21.2% and 6.3%~18.7% of total runoff amount and soil loss; while 29.1%~51.3% and 25.6% ~39.3% for rill erosion zone and 32.7% ~69.5% and 37.2%~78.1% for ephemeral gully erosion zone.(3) Effects of upslope runoff on soil erosion characteristics of downslope under different rainfall regimes were studied. The effects of upslope runoff on soil erosion characteristics of downslope were influenced by rainfall regime. Upslope runoff induced soil loss at rill erosion zone increase 30%, 26% and 23% under RR1, RR2 and RR3, respectively; while 41%, 38% and 33% for ephemeral gully erosion zone. For RR1, the increments of upslope runoff on soil loss at ephemeral gully erosion zone were much larger than those at rill erosion zone; while there were opposite trends for RR2 and RR3. The increment coefficients of upslope runoff on soil loss at rill erosion zone and ephemeral gully erosion zone were increased with the increase of P and I30.(4) The effects of rainfall regime on rill erosion and its morphological characteristics were analyzed. Soil loss at rill erosion zone and sheet + rill combined erosion zone caused by RR1 occupied 42.7% and 46.0% of total rill erosion, respectively; while 54.2% and 50.7% for RR2, and only 3.1% and 3.2% for RR3, respectively. Rill density, rill dissection degree and rill width-depth ratio of rill erosion zone and sheet + rill combined erosion zone were significantly different under three rainfall regimes, and decreased in order of RR1, RR2, and RR3. Rill dissection Degree was the best derivative morphological indicator to evaluate rill morphology.(5) The effects of rainfall regime on soil and water loss at hillslope with ephemeral gully were explored. The mean annual runoff and soil loss at hillslope with ephemeral gully caused by RR1 were accounted for 59.4% and 69.9% to annual runoff and soil loss, respectively, followed by 24.4% and 20.7% for RR2, and then RR3, only 16.3% and 9.4%. Rainfall and runoff curves under RR1 appeared single peak, while multi-peak for RR2 and RR3. For RR1, the time that peak flow discharge occurring was consistent with the time of peak values of rainfall intensity; however, the times were lagged behind that of peak values of rainfall intensity for RR2 and RR3.(6) Regression equations between soil loss and PI30 under different rainfall regime and erosion patterns were fitted. According to different erosion patterns, equations between soil loss and PI30 for sheet erosion zone, sheet + rill combined erosion zone and hillslope with ephemeral gully under different rainfall regimes were established, respectively. Compared with the equation without rainfall regime classification, the equations with rainfall regime classification had higher prediction precision. The prediction precision of the equations with rainfall regime classification increased to 20.5%.