The Influences Of Orchard Expansion On Catchment Hydrological Connectivity And Sediment Yield: Scenario-based Simulation

Good water and heat conditions in the red soil region of southern China provide a favorable planting environment for orchard.The orchard industry has been vigorously developed with the support of the state and local governments,and the large-scale expansion of orchards has brought considerable economic income to local residents.However,the homogenization of catchment landscape pattern caused by orchard expansion may also have a negative impact on the local ecological environment.The destruction of native vegetation by orchard expansion may change the hydrological connectivity of the catchment,which in turn may affect the erosion and sediment yield in the catchment.Therefore,simulating future orchard expansion scenarios and analyzing the impact of orchard expansion on hydrological connectivity and sediment yield can provide a reference for catchment management and is important for achieving a win-win situation for the regional ecology and economy.In this study,the CA-Markov model was applied to simulate the land-use distribution patterns of different future orchard expansion scenarios（the business as usual,restricted orchard expansion,and extreme expansion scenarios）.The effects of orchard expansion on the hydrological connectivity were analyzed.We further estimated the soil erosion modulus and sediment yield in the catchment,and quantitatively analyzed the effects of changes in the hydrological connectivity on sediment yield.The impact of hydrological connectivity on erosion and sediment yield was quantified.The following conclusions were obtained from this study:（1）From 2001 to 2015,the arable land area in the Yangxinjiang catchment decreased by28.24 km²,while the forest area and orchard area increased by 3.32 km² and 23.22 km²,respectively.The CA-Markov model was used to predict the land-use distribution patterns under different orchard expansion scenarios in the Yangxinjiang catchment.The orchard area in the business as usual scenario increased from 8.09%to 17.99%,mainly converted from forest land.In the restricted expansion scenario,the arable area is protected and the orchard expansion area increased from 46.03 km² to 80.24 km².As for the extreme expansion scenario,the orchard area reached 40.37%and the arable area and forest area decrease significantly.In addition,the areas of orchard expansion under the three scenarios were mainly distributed in areas with gentle slopes and low terrain.（2）Land-use changes significantly affected the hydrological connectivity of the catchment,with the mean value of index of connectivity（IC）decreasing from-3.90 in 2001 to-4.00 in2015.The variation of soil erosion ranged from 1.96×10⁵ to 6.35×10⁵ t a^-1 and sediment yield decreased from 1.40×10⁵ t a^-1in 2001 to 9.33×10⁴ t a^-1 in 2015.By combining the IC and RUSLE models to estimate sediment yield in the basin from 2001 to 2015,and comparing them with the measured values in the catchment.It was found that the Nash coefficient is 0.43,the Willmott’s Index of Agreement is 0.68,and the coefficient of determination is 0.53 of the two fits,indicating that the estimation results are acceptable.（3）The hydrological connectivity of the catchment varied significantly among different orchard expansion scenarios.Compared with the basic scenario,the mean IC values in the business as usual scenario,restricted expansion scenario and extreme expansion scenario increased by 9.25%,10.5%and 20%,respectively.A further hierarchical comparison of the hydrological connectivity of the watershed reveals that the areas with high increase in IC in the three orchard expansion scenarios generally correspond to areas where forest is converted into orchard.Furthermore,areas with high decrease in IC are generally distributed in areas where arable land is converted into orchard.These areas with large changes in hydrological connectivity are mainly located near the main stream and in some areas in the upper part of the Yangxinjiang catchment.The average sediment yield of the natural development scenario,the constrained expansion scenario and the extreme expansion scenario were 1.63 t ha^-1 a^-1,1.68 t ha^-1 a^-1 and 1.64 t ha^-1 a^-1,respectively.the sediment yield of the extreme expansion scenario was only 0.7%higher than that of the natural development scenario.In summary,the combination of IC index and RUSLE model can accurately simulate watershed sediment yield,and hydrological connectivity has great potential to quantify sediment transport processes.The expansion of orchards in the catchment contributes to the increase in hydrological connectivity,but the response of sediment yield to orchard expansion is not sensitive due to the decrease in soil erosion.This result suggests that a moderate orchard expansion（no more than 40%）may not cause a significant increase in catchment sediment yield.