Impact of Land Use on Frequency of Floods in Yongding River Basin, China
As the debates surrounding the negative influences of flood control using dams or reservoirs on the eco-environment become fierce, non-structural flood control measures like land use change gain more attention. This study researched the effect of integrated and single land use changes on three floods at small, medium and large scales, respectively, in Yongding River basin. A SWAT (Soil and Water Assessment Tool) model was used to simulate the effect of integrated and single land use changes on
... loods of different scales. The single land uses were set as S1, S2, S3 to represent the agricultural, grass and construction land changes. The results showed that: (1) the integrated land use changes reduced the small flood, the medium flood and the large flood by 14%, 13% and 5%; (2) the land use management functioned most effectively on medium-scale floods and least effectively on large-scale floods; (3) S1 decreased the medium floods optimally by 24% with a 7-day maximum runoff volume as the indicator and by 29% with a 1-day maximum flood discharge; (4) S2 reduced the medium floods optimally by 21% with runoff depth volume as the indicator; (5) S3 increased the medium floods optimally by 15% with a 1-day maximum flood discharge as the indicator. Non-point flood alleviation of land use management fits this new trend. The concept of non-point flood alleviation is centered on using surface measures to assist point measures (dams and embankments). By changing the land use, flood protection of the basin can be more effective while the focus of non-point flood alleviation is to retain rainfall on the surface and in soil pores. Land use plays a very important role in the hydrological processes of a certain basin  . Plenty of researches and experiments have proved that different land uses can lead to changes in the runoff process. Beven et al.  compared the land use and the floods happening in the same period with the land use changed in the past to find the historical connection. Woldeamlak  analyzed the pattern of the runoff in northwestern highland Ethiopia with the dynamics method and confirmed that the decrease of forest, the increase of the agriculture land, the grassland degradation and artificial afforestation would all be crucial factors having an influence on the runoff process. McIntyre  testified the connection between floods and land use at the regional scale. Li et al.  quantified the effects of land use change on flood peaks and volumes in Daqinghe River basin with a multiple-linear regression analysis method. Now it has been established that the variable of land use can change the flood pattern, whether it can achieve the aim of flood control by increasing some sort of land use which benefits rainfall retention still remains to be explored. This flood control measure is different from the punctate measure of dams which is called non-point flood alleviation. The main effect of the land use on runoff/flood is to reduce the rainfall energy by interception, sink filling, transpiration, evapotranspiration and infiltration  . Nie et al.  demonstrated in the upper San Pedro watershed that construction land and the decrease of forests would cause surface runoff to increase. A similar conclusion was made by Lin  in Willow Basin, Canada. The negative changes aforementioned could lead to the degradation of the soil which can lower the infiltration, causing poorer connectivity and impairing the water-holding capacity, thus increasing the flood risk  . According to Feng , grassland can delay the runoff three-fold compared to bare land. The average speed of the runoff on grassland is only 40% of that on bare land. The root system of the grass increases in surface soil voids. The grass covering the surface can also ease the impact of rain drops hitting on soil and compromise the integrity of the soil surface, while also increasing the roughness of the surface soil, so the runoff coefficient becomes significantly smaller  . Fan et al.  compared bare land, grassland and woodland runoff over years with a monthly distribution. Runoff of bare land, grassland and woodland accounted for rainfall over the same period of 18.6%, 5.1%, 3.7%, respectively; and the uniformity coefficients of the bare land, grassland and woodland for the distribution of monthly runoff were 0. 99, 0.74, and 0.69 . Therefore, planting trees and grass reduces runoff and improves the uniformity of the distribution of runoff. As for agricultural land, there has been plenty of studies proving its benefit in preventing floods. Sujono  showed through experiments that some measures of agricultural planting can reduce surface runoff. Evans et al.  showed that certain measures at autumn sowing can lower the frequency of muddy floods. The phenomena can be explained by the fact that with the measures carried out, the soil structure is changed and the land surface is uneven, so the rain either goes to the deep soil layers through the gaps or fills in the depressions  . In addition, the residue covering can protect the land surface from being hit by the rain drops, and slows down the speed of infiltration  . Despite the researches aforementioned, the effects of the land use change on the runoff/flood still remain unclear, especially at basin scale  . For some basins, the flood process is influenced by several factors, such as rainfall, elevation, soil depth, and water steepness  , and land use change depends on the former type, the change type, the location, the change time and the like . Table 1 lists some researches trying to distinguish the effects with various methods.