The Research Of Watershed Runoff And Sediments Variation Toward To The Soil And Water Conservation Terrace Measure

It was widely focused by the nation and society that the runoff of the partial importantriversreducesharply. The scientific issue that the influence mechanism of the soil and waterconservation measures,especially the terrace measure on the stream runoff and sediment wasnot clear.Based on the further improvement of “Watershed Self-response Theory”,this thesisstudied the terrace destroyed process under the extreme rainstormcondition as thebreakthrough point of the research. Utilizing the physical model and mathematical model asthe research method, this thesis constructed the1:10normality terrace scale mode withthesimilarity of rainfall,runoff,erosion andsedimentstransport. The thesis studied the erosionprocess of the typical terrace under the extreme rainstorm conditions.Based on the theresearch, according to the newly developed488version of SWAT model embedded theterrace module, the thesis studied the effect of the large-scale terrace measure on the runoffand sediment changes in the watershed with water shortage and heavy sediments. The mainconclusions were as follows:(1) This thesis enriched and improved the “Watershed Self-response Theory” further,raised the watershed elements interactions frame diagram, researched the special rainfallinfluence on thewatershed runoff and sediments. Based on the theory, this thesis analyzed thecharacter of the2013Yan’an extreme rainstorm. The results revealed that the maximum10minutes,30minutes,60minutesrainfall intensity was0.7mm/min,0.62mm/min,0.4mm/minrespectively.The rainfall intensitywas not large. However,the maximumdaily,3days,5days,6days,7days, monthly,3months, yearly precipitation was87.8mm,141.1mm,223.6mm,269.7mm,272.6mm,568mm,761.3mm,959.1mm respectively. Specifically,the maximummonthly precipitation was beyond the ten thousand years rainstorm standard. As a result themassive erosion disaster happened in YanRiver basin,but the river water and sediment weresmall.The maximum daily runoff of Yan’an station and Ganguyi station was the5yearsfloodstandard.Some new river features thatlow rainfall intensity, long duration, severe erosionhazard, and the relatively small river water and sediment,appeared in theYan River basin. Thenew situation indicated that the watershed underlying surface, especially the terraces on theimpact of watershed water and sediment basin was worth studying.(2)According to the actual terraces destruction process in extreme storm conditions wasdifficult to quantitative analysis, based on the development of small watershed entitiessimulation theory and technology, this thesis studied the typical terrace measure variationtoward to the extreme rainstorm erosion processby constructing terraces Scale Model andverification.And the results indicated thatthe destruction process could besimulated bytheequivalent-erosion-process of the scale model artificial.According to the previous similarity scale this thesis constructed the1:10normality terrace scale model,simulating the extremestorm of2013under the similar conditions terrain, the results indicated that the equivalent-erosion-precipitation was636mm, the equivalent-rainfall-intensity was1.25mm/min，andthe equivalent-rainfall-duration was508min. The results showed that location of theefficientagricultural terracesaffected the terraceerosion during the extreme rainstorm. Under theexperiment conditions,the bottomterraceserosion was server than the upper, the maximum soilerosion modulus was more than5.4(104t/km2), and the average soil erosion modulus wasabout2.4(104t/km2) in the typical8steps terrace cropland.The survey results showed that theterrace erosion intensitywas about twice the area average erosion intensityin storm conditions,the reasons was worthy furtheranalysis. When constructing terraces in loess hilly regions, thedifferent design criteria of the upper and lower terraces at different locations should be takeninto consideration, to improve thebenefits of soil and water conservation.(3) In order to take a further research of theterracemeasures effect on the river runoffand sediments process, especially aiming at the problems thatthe rapid reduction of riverrunoff and sediments,the ecological base flowswas shortage and the runoff and sedimentschanging variationresearch was still weak,this study chosethe488thSWATmodel embeddedthe newly developed terracemodule, toresearch the runoff and sedimentschangingmechanismand control measures’optimal allocation in the highly human-controlled Yan river, which wasshort in water but rich in sand.Based on updating the model soil and weather database,thisthesis selected the year from1957to1962as the model calibration period, the main parametervalues of river runoff and sediments was given. Using the NS coefficient and the PBIAS asevaluation index,this study used theYan’an and Ganguyi stationmeasured datafrom1963to1969, the human activities was few,to verify the model.The results showed that daily scalesimulation results and the measured results had the same tendency, but could not meettheprecision requirement. While on monthly scale, both tendency and precision requirementcould meet the modelaccuracy requirement, with theNS and PBIAS were ranged from0.55~0.88and-16.2%~22.8%respectively. This study showed that in highly human-influenced Yan river,with limited water and abundant sediments, it wasfeasible to usethe488thSWATmodel embedded the newly developed terracemoduleto simulate thewatershed runoffand sediments process.(4) The research ofthe terrace influence on the watershed runoff and sediments. Usingthe constructed mathematical model studied the effect of the terrace measures on the streamflow and sediment. And the results showed that,the terrace measures of Yan river watershedhad the function of “flood reducing and base flow supplying”,which indicated that the terracemeasures could reduce the runoff in the flood season, increasethe base flow and guarantee theriver ecological flows.This thesis raised the concept of the terrace “flood reducing to the base flow supplying”,which was the ratio of the total runoff reduction amount to the supplying runoffamount of theterrace to the river, and quantitatively gave the value of the “flood reducing to the base flowsupplying ration” of Yan river wasabout5.58, which showed that1m3water could be suppliedto the river while5.58m3water stored by the terrace.The constant flow of the most gullieswas the proof. Compared to the baseline period(1956-1969), the2005and2011watershed terrace couldreduce the Ganguyi station flow up to397.3(104m3) and578(104m3), and supply the riverflow95.7(104m3) and129.5(104m3). The2005and2011watershed terrace could reduce theGanguyi station sediments yield271(104t) and374.2(104t), the sediments yield reduced byper area terrace was1.08(104t/(km2.a)) and1.28(104t/(km2.a)) respectively.(5) This thesis studied the rational distribution of MaJiaGou watershed terrace. Theresults conclude that the distribution of terraces obey the principle of “up is better than down”.It means that the terraces distributed in upstream are more beneficial than thatindownstreamin the aspect of reducing sediments. It was also applicable in the same section.On the basis of fully considering the duty of watershed such as providing fresh water,reducing sedimentsbenefits, economic benefits, ecological benefits and social benefits, it wasfit for this watershed to develop terrace high efficient agriculture and the terraces ought to beplaced on upper of watershed downstream. The calculating results also showed that the perarea terrace sediments reducing amount can reached to2520t/km2, and the rationaldistribution of watershed terrace can improve20%of the terrace sediments reducing benefits.The results could provide technical support for the watershed soil and waterloesscomprehensive treatment.