Simulation of Flow and Agricultural Non-Point Source Pollutant Transport in a Tibetan Plateau Irrigation District

Yuqing Li,Zuhao Zhou,Chongyu Xu,Kang Wang

doi:10.3390/w11010132

Abstract

Flow and transport processes in soil and rock play a critical role in agricultural non-point source pollution (ANPS) loads. In this study, we investigated the ANPS load discharged into rivers from an irrigation district in the Tibetan Plateau and simulated ANPS load using a distributed model. Experiments were conducted for two years to measure soil water content and nitrogen concentrations in soil and the quality and quantity of subsurface lateral flow in the rock and at the drainage canal outlet during the highland barley growing period. A distributed model, in which the subsurface lateral flow in the rock was described using a stepwise method, was developed to simulate flow and ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) transport processes. Sobol’s method was used to evaluate the sensitivity of simulated flow and transport processes to the model inputs. The results showed that with a 21.2% increase of rainfall and irrigation in the highland barley growing period, the average NH4+-N and NO3−-N concentrations in the soil layer decreased by 10.8% and 14.3%, respectively, due to increased deep seepage. Deep seepage of rainfall water accounted for 0–52.4% of total rainfall, whereas deep seepage of irrigation water accounted for 36.6–45.3% of total irrigation. NH4+-N and NO3−-N discharged into the drainage canal represented 19.9–30.4% and 19.4–26.7% of the deep seepage, respectively. The mean Nash–Sutcliffe coefficient value, which was close to 0.8, and the lowest values of root mean square errors, the fraction bias, and the fractional gross error indicated that the simulated flow rates and nitrogen concentrations using the proposed method were very accurate. The Sobol’s sensitivity analysis results demonstrated that subsurface lateral flow had the most important first-order and total-order effect on the simulated flow and NH4+-N and NO3−-N concentrations at the surface drainage outlet.

Highlights

The use of fertilizers in irrigation districts is considered as one of the most common sources of environmental pollution [1,2]
The mean Nash–Sutcliffe coefficient value, which was close to 0.8, and the lowest values of root mean square errors, the fraction bias, and the fractional gross error indicated that the simulated flow rates and nitrogen concentrations using the proposed method were very accurate
Water flow and transport processes were simulated in irrigation subdistricts with different parameters related to the irrigation events and the size of the irrigation subdistricts

Summary

Introduction

The use of fertilizers in irrigation districts is considered as one of the most common sources of environmental pollution [1,2]. Water 2019, 11, 132 model CASC2D (Colorado State University, Fort Collins, CO, USA) [4], dynamic watershed simulation model [5], and soil and water assessment tool (SWAT) model (United States Department of Agriculture, Washington, DC, USA) [6,7]. The modules in these models have been modified to reflect the impacts of irrigation practices and agricultural managements on migration processes of water and fertilizer in irrigation zones. The terrain elevation was corrected in the spatial grid unit to describe the irrigation drainage system [9,10]

Objectives

Methods

Results

Discussion

Conclusion