Research On The Changes Of Land Use Landscape Patterns And Their Effects On Runoff In Jinghe River Watershed

The Loess Plateau is one of the most serious areas of soil erosion all over the world with low forest coverage rate,scarce water resources and fragile ecological environment.In recent decades,major forestry ecological projects such as afforestation in barren hills and Grain for Green have played an active role in controlling soil erosion and improving the ecological environment.However,the practice of these projects has greatly changed the land use/cover and landscape pattern and thus deeply affected the hydrological cycle process and water resources security in the region.Therefore,the studies of the relationship between land use/cover changes and runoff would not only have important theoretical value for understanding the mechanisms of impacts of land cover and landscape pattern changes on hydrological process,but also provide valuable guidance for rational planning of soil and water resources and ecological restoration in a given watershed.In this study,land use and landscape pattern change and their effects on runoff were studied by using Arcgis,Fragstats and statistical softwares in Jinghe river watershed,which is located in the centre of the Loess Plateau.First,the characteristics of the changes in precipitation and runoff in the Jinghe river watershed were analyzed during 1966-2010 years,the inflection points of runoff were determined and the effects of climate change(mainly precipitation)and human activities on runoff were analyzed.Then,the characteristics of the changes in land use/land cover and landscape pattern in Jinghe river watershed during1980-2015 years were studied,and it's impact of the changes on runoff were also analyzed.Finally,the hydrological processes under different forest coverage rate and ages in Larix principris-upprechtii plantation were simulated and analyzed in the upper reaches of Jing River,aiming to provide scientific guidance for the prediction of long-term effect of reforestation project on runoff.Main conclusions have been made as follows:(1)Changes in precipitation and runoff in the Jinghe river watershed were characterized by means of Mann-Kendall non-parameter test,Sen slope estimation and Mann-Kendall ranktest,the results showed that during 1966-2010,precipitation showed no significant downward trend,with an average annual decrease of 0.6 mm;however,annual runoff depth showed a significant downward trend,with an average annual decrease of 0.7 mm.Both using the methods of the cumulative offset curve and the double cumulative curve,the years of 1986 and1997 were determined as the two inflection points of the relationship curve between precipitation and runoff,and the whole study period could be divided into three periods:1966-1985,1986-1996 and 1997-2010.The period of 1966-1985 was used as the baseline period,and contribution rate of climate change to runoff decreased from 60.7% in 1986-1996 to 16.8% in 1997-2010,but contribution rate of human activities increased from 39.3% to83.2%,indicating human activity had become the dominant factor to decrease runoff.(2)Characteristics of changes in land use/land cover were studied by using Arc GIS 10.2during 1980-2015 years in Jinghe river watershed,it showed that during 1980-2015,cultivated land and grassland were the two dominant landscape types,accounted for over 87% of total area of the watershed;forests was followed with a ratio of 8.98%-9.97%;water,construction land and unused land were accounted for less than 2.90% together.The spatial distribution of cultivated land,grassland and forests showed a characteristics of mosaic layout.It could be seen that land use transfer was mainly between cultivated land,forests,grassland and construction land in the watershed during 1980-2015.Especially in the early stage of reforestation project(2000-2005 years),the total net change rate of watershed(Sa=1.11%),transfer trend(Pt=0.71)and net change speed(Rt=0.22%)were largest,suggesting the conversion degree between land-use types was the most severe.During 1995-2015,cultivated land decreased by 949.04 km2,mainly to forests,grassland and construction land;forests increased by 441.09 km2,mainly from cultivated land and grassland;grassland increased by223.54 km2,mainly from cultivated land and forests;construction land increased by 307.37km2,mainly from cultivated land;and water area changed little.(3)Characteristics of changes in landscape pattern were studied by using Fragstats 4.2,it showed that during 1980-2015,landscape pattern of the watershed tended to be characterized by deceasing connectivity and increasing fragmentation and heterogeneity.At the patch-classlevel,forests was the most fragmented landscape type.Since 1995,fragmentation degree of cultivated land and forests deepened,while fragmentation degree of grassland weakened.Cultivated land and grassland had greater dominance,but both showed a trend of increasing first and then decreasing.Affected by human activities,the shape of patches in cultivated land,forests and water tended to be simplified,and the shape of patches in grasslands and construction land tended to be complicated.(4)Relationship between landscape pattern and runoff was studied with Spearman correlation analysis,it showed that largest patch index(LPI),contagion index(CONTAG)and area-weighted mean shape index(AWMSI)were significant or extremely significant positive correlation with runoff,indicating the increase of dominance degree,aggregation degree and shape complexity of patch were conducive to the formation of runoff.However,significant or extremely significant negative relationships between runoff and number of patches(NP),patch density(PD),landscape division index(DIVISION)and Shannon's evenness index(SHEI)meant fragmentation and heterogeneity of landscape contributed to impede the development of runoff.(5)Take Larix principis-rupprechtii plantation as an example,the impacts of forests change on runoff was analyzed by using SWIM model in the upper reaches of Jing river.The results showed that evapotranspiration increased with the increase of coverage rate of the plantation,but annual runoff depth,runoff depth in dry season and runoff depth in wet season showed downward trends.With the increase of forest ages of Larix principis-rupprechtii plantation,evapotranspiration increased from young forest to near-mature forest,and it remained basically unchanged after the near-mature forest.While annual runoff depth,runoff depth in dry season,and runoff depth in wet season showed downward trends from young forest to near-mature forest,and the reduction rate of those gradually decreased after the near-mature forest.