Impact of urbanization on hydrological processes under different precipitation scenarios

According to analysing the trends of land use changes in the upper reaches of Minjiang River in the past 30 years and precipitation in the last 50 years, nine types of simulation scenarios were constructed for different precipitation conditions and urbanization development processes. Based on the “five sub-basin selection principles” and “two simulation results evaluation indicators” proposed, the paper studied the influence of the urbanization process on hydrological processes under different precipitation conditions using the SWAT model. The primary conclusions are as follows: (1) the simulation results under the two kinds of land use transfer scenarios show the same laws: (a) when forest land (or grassland) is transferred to urban land, actual evapotranspiration (ET), soil water content (SW), amount of water percolating out of root zone (PERC) and groundwater contribution to streamflow (GW_Q) show a decreasing trend, and the reduction in watershed hydrological indexes is manifested as “high precipitation > average precipitation > low precipitation”. Moreover, surface runoff (SURQ), water yield (WYLD) and annual runoff show an increasing trend, and the increment in SURQ shows “high precipitation > average precipitation > low precipitation”, while the increment in WYLD and the simulated annual runoff show “low precipitation > average precipitation > high precipitation”. (b) Through analysis of the contribution of unit proportion transfer (CUPT) of watershed hydrological indicators, “SURQ > PERC > GW_Q > ET > SW” is observed in all precipitation scenarios. (2) Comparing simulation results between the two kinds of land use transfer scenarios: the CUPT variations of ET, SURQ and WYLD and the contribution of unit area transfer variations of daily flood peak and annual runoff both show “forest land transfer to urban land > grassland transfer to urban land”. Finally, two special phenomena observed in the analysis of the simulation results were discussed. The study results can provide a scientific basis for urban planning and construction for reducing the impact on urban flood.