The Effect of Urbanization and Farmland to Forests Program on Surface Runoff

Over the past twenty years, significant areas of agricultural and forest land have been converted into construction land due to urbanization in China. Additionally, the national Green policy has led to the reforestation of substantial portions of agricultural land. This study employed remote sensing technology and Soil Conservation Service (SCS) Curve Number (CN) model to investigate land use types and runoff. Multiple regression models were then used to analyze the annual runoff coefficient for agricultural, forest and construction land. The results revealed the following, (1) Agricultural land, forest land, and construction land were the dominant land use types, accounting for 95.61%~98.65% of the study areas. The area of agricultural land declined linearly, shrinking by 40.58 km² every five years; while forest and construction land increased linearly 24.229 km² and 18.187 km², respectively, every five years. (2) Changes in the CN values were driven by land use shifts among agriculture, forest and construction land. Before 2016, the maps with a CN value of sixty-nine were replaced by those with a CN value of fifty-five, reflecting the conversion of agricultural land to forest land under the conversion of farmland back to forests project. Meanwhile, the number of maps with a CN value of seventy-five increased, aligning with the process of urbanization. (3) The annual runoff coefficient declined as probability increased. The runoff coefficient decreased linearly with the rate of urbanization, with slopes of straight-line − 0.110 (for P10), -0.130 (for P30), -0.136 (for P50), -0.142 (for P70), and − 0.151 (for P90). A 10% increase in urbanized rate was found to raise the annual runoff coefficient by 1.1–1.51%. The annual runoff coefficient impacted by urbanization ranged between 0.0349 and 0.0486, accounting for 4.79%~7.81% of total. The runoff coefficients for agricultural land, forest land, and construction land were 0.8015, 0.6273, and 0.8279, respectively. During the study period, the contribution of construction land to runoff coefficient increased from 7.98 to 33.06%, while the contribution from forest land increased from 0.1526 to 0.3605. In contrast, the contribution of agricultural land to the runoff coefficient declined from 0.5074 to 0.0651, with an average annual decrease of 2.21%.