Responses of Rainfall-Runoff Processes and Nitrogen and Phosphorus Exports to Different Landscape Pattern in the Liangzi Lake Watershed.

The landscape pattern controls the energy flow and water sediment process of the watershed, which is an important factor affecting nitrogen and phosphorus loss in small watershed. In order to understand the responses of rainfall-runoff processes and nitrogen and phosphorus exports to landscape pattern, two typical small watersheds, Shuangxi (agroforestry catchment) and Shiqiao (forest catchment) in the upper watershed of Liangzi Lake area of Hubei Province, were selected as the research objects. The water quality and quantity at the outlet of the watersheds were continuously monitored from January to December and a secondary rainfall event in 2016. The results showed that the landscape patterns had a significant impact on responses of rainfall-runoff processes. Compared with Shuangxi, the runoff process in Shiqiao small watershed showed a trend of shorter convergence time, faster runoff response speed and larger runoff modulus. The monthly concentrations of total nitrogen (TN) and total phosphorus (TP) in Shuangxi small watershed were in the range of 1.29~4.28 mg/L and 0.04~0.22 mg/L, which were higher than those of Shiqiao (TN:0.62~1.90 mg/L; TP:0.02~0.11 mg/L), respectively. Nitrate nitrogen (NO^-³-N) was the major form of TN in the two small catchments, accounted for 51.8%~88.2%, but the proportion of NO^-³-N to TN in Shuangxi small watershed was higher than those of Shiqiao. The concentrations of ammonium (NH⁺⁴-N) and nitrite nitrogen (NO^-²-N) remained at a low level and exhibited no obvious trend. The emission fluxes of TN and TP in Shuangxi small watershed were in the range of 1.83~20.03 g/s and 0.04~1.45 g/s,which were higher than those of Shiqiao (TN:0.62~10.01 g/s; TP:0.01~0.54g/s), respectively. It was also found that there was a significant positive correlation between the TN and TP emission flux and runoff in the same period. The annual loss intensity of TN and TP in Shuangxi small watershed were 3.68 t/(km² · a) and 0.20 t/(km² · a), which were 1.32 and 1.30 times greater than those of Shiqiao, respectively. The losses of TN and TP in both small watersheds mainly occurred between April and July with frequent rainfall events, accounting for more than 70% of the annual total losses. In conclusion, different landscape patterns had a significant impact on the losses of TN and TP. Accordinglly, rational fertilization to reduce anthropogenic nitrogen and phosphorus loss and optimization of land use to store water and preserve soil are scientific measures to improve the water environment in the watershed. The study could provide more insights for reducing agricultural non-point source pollution in Liangzi Lake region and similar other regions. [ABSTRACT FROM AUTHOR]