Effect of surface heterogeneity on hyper-resolution simulation of soil moisture

<p>Due to the land surface complexity, soil moisture immensely varies both spatially and temporally. However, the combined effects of land surface complexity and key hydrological processes (e.g., subsurface lateral flow) on fine-scale soil moisture heterogeneity remain elusive due to the scarcity of observations. Benefit from improvements in hyper-resolution land surface modeling, it provides an unprecedented opportunity to investigate the fine-scale soil moisture heterogeneity over a large region. Here, we use the Conjunctive Surface-Subsurface Process model version 2 (CSSPv2), which considers subsurface lateral flow, to perform hyperresolution (100-m) simulations over ten selected regions with different climate. We find that the heterogeneities of vegetation, soil texture, precipitation or their combinations increase soil moisture heterogeneity significantly (p<0.01). If only the topography heterogeneity presents, subsurface lateral flow increases the soil moisture heterogeneity significantly (p<0.01). However, the effect of subsurface lateral flow has been reduced by combining topography heterogeneity with other surface heterogeneities, with a few regions showing decreased soil moisture heterogeneity mainly because of the combined effect of subsurface lateral flow and soil texture heterogeneity. This study suggests that soil texture heterogeneity does not necessarily interact synergistically with physical processes (e.g., subsurface lateral flow) for increasing soil moisture heterogeneity, although they can increase the heterogeneity separately.</p>

Subsurface Preferential Flow Enhances Hydrological Connectivity in the Shale Hills Catchment: Perspective from Wavelet-based Analysis

<p>Preferential flow (PF)-dominated soil structure is often considered a unique system consisting of micropores and macropores and thus supposed to provide dual-pore filtering effects on hydrological signals, through which smoothing effects are likely to be stronger for matrix flow and weaker for PF via macropores. By using time series of hydrological signals (precipitation, canopy interception, throughfall, soil moisture, evapotranspiration, water storage in soil and groundwater, and catchment discharge) propagating through the Shale Hills Catchments and representative soil series, the filtering effects of the catchment and soil profiles were tested through the wavelet analysis. The hypothesized dual-pore-style filtering effects of the soil profile were also confirmed through the coherence spectra and phase differences, rendering them applicable for possible use as “fingerprints” of PF to infer subsurface flow features. We found that PF dominates the catchment’s discharge response at the scales from three to twelve days, which contributes to the catchment discharge mainly as subsurface lateral flow at upper or middle soil horizons. Through subsurface PF pathways, even the hilltop is likely hydrologically connected to the valley floor, building connections with or making contributions to the catchment discharge. This work highlights the potential of wavelet analysis for retrieving and characterizing subsurface flow processes based on the revealed dual-pore filtering effects of the soil system.</p>

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

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.

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

Flow and transport processes in soil and rock play a critical role in agricultural non-point source pollution (ANSP) 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 involving detailed descriptions of flow and ANPS transport and transformation processes in the soil and rock. Experiments were conducted for two years to measure soil water content and nitrogen concentrations and the quality and quantity of 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 was described using a step-wise method, was developed to simulate flow and ammonium nitrogen and nitrate nitrogen transport. Sobol’s method was used to evaluate the sensitivity of simulated flow and transport processes to 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 channel represented 19.9–30.4% and 19.4–26.7% of the deep seepage, respectively. The mean Nash-Sutcliffe coefficients, root mean square errors, and cumulative deviations between the measured and simulated flow rates and NH4+-N and NO3--N concentrations at the surface drainage canal outlet were 0.694, 0.081, and 0.242, respectively, indicating that the proposed method can effectively describe the hydrological and ANPS pollution migration in the plateau irrigation zone. The Sobol’ sensitivity analysis results demonstrated that subsurface lateral flow had the most important first order and total effect on the simulated flow and NH4+-N and NO3--N concentrations at the surface drainage outlet.