Study On The Erosion Feature And Control Effects Of Typical Terraces Under Extreme Rainstorm

With the implementation of the policy of converting slope farmland into forest and grassland, there is an increasing conflict between food security, water security, and cultivated land resources, so it is quite popular to develop terrace highly efficient agriculture in loess hilly and gully region. So far, the ways of terrace highly efficient agriculture mainly contain terrace-field, terrace-greenhouse, terrace-orchard, plain field agriculture etc. Owing to the impacts of global climate change,the contradiction between aridity and erosion in loess hilly and gully region become worse,the frequency of extreme weather and climate events especially extreme rainfall become intensive,and soil erosion have new feature. As an important way of controlling soil and water loss and developing agriculture, terraces are always highly efficient, but terraces also change the underlying surface. Under the change of runoff producing precipitation and confluence and extreme rainstorm, terraces are often seriously damaged. Based on the extreme rainfall happened in Ansai county Yan’an city in 2013, this study monitored the soil and water erosion condition of typical terraces in loess hilly and gully region, analyzing the erosion law and influence factors of terraces, and puts forward terraces erosion controlling suggestions. The main conclusions are listed as follows:1.The characters analysis of extreme precipitation under the condition of climate change in the study area. The annual average rainfall is 505.3 mm, but there was an extreme rainstorm in 2013 in study area, which not occur even in a hundred years. Based on analyzing the meteorological data from China meteorological and scientific data sharing net and Ansai weather bureau, the results showed that the maximum 10-minute rainfall intensity was a 1-year rainstorm recurrence, the maximum 20-minute rainfall intensity was a 2-year rainstorm recurrence, the maximum 30-minute and 60 minute rainfall intensity were both 5-year rainstorm recurrence, the maximum 3-hour and 6-hour rainfall intensity were both 20-year rainstorm recurrence, the maximum 1-day rainfall intensity was a 14-year rainstorm recurrence, the maximum 1-month rainfall intensity was a 10000-year rainstorm recurrence, whole-year rainfall intensity was a 500-year rainstorm recurrence in 2013. The 2013 rainfall has the feature of low rainfall intensity, but long rainfall duration and high frequency. The 2014 rainfall in rainy period in study area was 372.5 mm, with erosive rainfall accounting for 80% of whole rainfall. The results indicated that under the condition of climate change, the events of extreme rainstorm was increasing, which had a serious threat to the development of terrace highly efficient agriculture.2.There was a big difference of runoff and sediment regulation effect from different ways of terrace highly efficient agriculture under extreme rainstorm. Under extreme rainstorm from June to August in 2013, due to the fact of lacking runoff and sediment regulation measures and confluence of runoff from the top to the bottom of multilevel terraces, the terrace-field was seriously erosive, with the average soil erosion modulus being 23 921 t/km2; after planting apple trees and building water cellar on terrace-field, under the regulation of water cellar and crops, the average soil erosion modulus of terrace-orchard was 2177 t/km2, decreased soil erosion modulus of 21 744 t/km2 compared with terrace-field; after building greenhouse on terraces, because of the high efficiency of rainwater collection on the surface of greenhouse, the average soil erosion modulus of terrace- greenhouse was 18269 t/km2, decreased soil erosion modulus of 6525 t/km2 compared with terrace-field. By building runoff and sediment regulation systems on terrace-greenhouse in 2014, rainfall runoff from greenhouse was intercepted and keeped in water cellar, what’s more, the motive power of soil erosion was removed. So there was only slight surface erosion on it, with anticorrosion effect was 100%.3.The erosion feature of typical terraces under extreme rainstorm. The erosion level of terrace-orchard were middle degree erosion. From the top to the bottom, the erosion level of terrace-field changed from light erosion,middle degree erosion, serious erosion, very intensive erosion to violent erosion,with the area of very intensive erosion accouting to 64.26% of whole area. The rainfall runoff from greenhouse surface occupy 84% to 92% of total rainfall runoff, which had an decisive role in soil erosion. The erosion level of terrace-greenhouse of every terrace was very intensive erosion.4.The suggestion of terraces erosion control. The series of terrace-field should not be over 5, and it’s better to build perfect erosion control measures, such as complete and strong bund, no bare land, building backslope terrace if conditions are allowed. When the series of terrace-field are over 5, it’s better to build runoff and sediment regulation measures such as discharge canal and water cellar. After building runoff and sediment regulation measures on terrace-greenhouse, the outside area of greenhouse should be made most of such as planting switchgrass to filter surface runoff, and drip irrigation systems should be installed inside of greenhouse. The rate of greenhouse surface to terrace area should be 0.4 to 0.5. As far as terrace-orchard is concerned, runoff-collecting surface and water cellar should be checked and maintained termly, and bunds should be dressed before rainy season comes. The litter and altar weeds are not suggested to be removed. In conclusion, under the condition of climate change, the design criterion of terraces should be improved.