Runoff generation processes estimated from hydrological observations on a steep forested hillslope with a thin soil layer

doi:10.1016/S0022-1694(97)00018-8

Journal of Hydrology

Volume 200, Issues 1–4, 15 December 1997, Pages 84-109

https://doi.org/10.1016/S0022-1694(97)00018-8 Get rights and content

Abstract

In order to understand runoff generation processes on a forested hillslope involving large heterogeneities, this study monitored runoff from a steep hillslope with a thin soil layer as well as matric potential within it and analyzed their responses to storm rainfall. A comparison of storm runoff responses from the study slope with those from two adjacent catchments, one of which includes it, showed that physical properties of the slope reflected the runoff characteristics: although no responses occurred in very dry conditions because of the absence of wet zones near the stream, the area contributing to storm runoff more rapidly extended to the whole slope due to its topographic properties. They also caused its steep hydrographs produced in the wettest condition where almost all the rainfall contributed to storm runoff. In this wettest condition, tensiometric responses near bedrock showed the vertical quick propagation of the rainfall pulse, and a good agreement of storm hydrograph simulated through a kinematic wave runoff model suggested that runoff from the slope was produced by a lateral flow on the bedrock receiving the quick propagation. In a transition process from dry to the wettest conditions, the development of the lateral flow producing smaller responses at the downslope end was estimated from decreasing of matric potential near bedrock from high negative to low values with increasing cumulative rainfall.

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Topography plays a crucial role in predicting rainfall-runoff processes under climate change, as it affects the spatial distribution of water flow pathways. However, understanding the potential impact of high micro-topography heterogeneity on runoff generation remains challenging. We conducted high-resolution monitoring of soil moisture, instantaneous water table, and subsurface runoff in two 3D plots with different soil thicknesses (66 cm vs. 35 cm) in the karst region of southwest China. The topographic features were characterized by the soil thickness, relative elevation, and topographic wetness index obtained from a 1 × 1 m grid. The topography of the shallow-soil plot (SSP) exhibited a higher level of complexity, with a greater variability (approximately 80.0 %) in soil thickness compared to that of the deep-soil plot (DSP; approximately 61.5 %). Additionally, the SSP was prone to forming a micro-topography characterized by a combination of exposed bedrock and soil cover. In DSP, the variation in soil moisture was strongly controlled by surface topography, with elevation contributing to 52 % and 81 % of the explanation for the soil moisture at 30 cm and soil–bedrock interface, respectively. In SSP, the contribution of topography to the variation in soil moisture did not exceed 31 %. The controlling effect of topography on runoff is highlighted in SSP, with the contribution rates of surface and bedrock relative elevation reaching 79% and 97%, respectively.The weaker water storage capacity of the SSP resulted in a higher volume of subsurface runoff (161 t vs. 116 t) and frequency of runoff events (2248 vs. 1994) compared to the DSP. Accordingly, this study emphasizes that the soil–bedrock spatial heterogeneity will control the relationship between soil moisture, subsurface runoff, and terrain, which will contribute to providing a basis for flood prediction and model simulation in regions with complex topography.
A field and modeling study of subsurface stormflow for Huanggou Hillslope
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The Huanggou Hillslope in China
The study of the threshold behavior and nonlinear characteristics of subsurface stormflow is not only essential for hydrology theory but also for flash flood disaster prevention. However, the formation mechanism and determination method of the thresholds have received little attention. Firstly, this study proposed the three-stage subsurface stormflow mechanism hypothesis. Secondly, based on in-situ observation, this study utilized the piecewise regression method and the soil moisture accounting method to analyze the threshold behavior and verify the three-stage mechanism hypothesis. And found that at the Huanggou Hillslope, the thresholds are related to the soil characteristic water content, and the sum of the thresholds of stratified runoff is approximately equal to the threshold of total runoff. Finally, this study developed the three-stage subsurface stormflow-based model (TSSM) and applied it to the Huanggou Hillslope and the Huanggou Watershed.
The results show that TSSM performed well, with NSEs of 0.82 and 0.67 in the calibration and verification periods of the Huanggou slope, and NSEs of 0.76 and 0.74 in the calibration and verification periods of the Huanggou Watershed, respectively. This study elucidated the three-stage subsurface stormflow mechanism and developed an effective simulation model, which contributes to increasing our understanding of three-stage subsurface stormflow and is beneficial for hydrologists to develop more realistic hydrological models.
Towards hydrological connectivity in the karst hillslope critical zone: Insight from using water isotope signals
2023, Journal of Hydrology
Citation Excerpt :
We also previously found that the surface and bedrock topographic wetness index were associated at a higher degree in the SSP (34 %) than in the DSP (3 %) (Figs. S2 and S3; Zhang et al., 2022a). A high degree of coupling between soil depth and bedrock means that they affect runoff generation in a combinatory manner, which agrees with findings in previous experimental plots and small watersheds (Freer et al., 2002; Hutchinson and Moore, 2000; Peters et al., 1995; Tani, 1997). Through further analyses in this study, we found that the spatial heterogeneity of bedrock topography caused differentiation of the hillslope confluence area, which further affected the formation of the spatial saturation zone (Fig. 9).
Hydrologic connectivity along the hillslope critical zone is essential for the transmission of water, solutes, and nutrients to streams. However, the influences of critical zone structure on hydrological connectivity are not adequately understood, especially for hillslopes with complex topography. We investigated the hydrological connectivity along different topographical positions of two adjacent dolomite hillslope plots in southwest China: a deep soil plot (average value: 66 cm) and shallow soil plot (average value: 35 cm). We characterized the dynamics of multiple-interface runoff and soil water based on observations of stable isotopes (δ¹⁸O and δD) and hydrometric monitoring, and computed the lc-excess, young water fraction (y_w), and hydrograph separation from June 2020 to July 2021. On both hillslopes, soil depth gradually increased from up-slope to down-slope, and the contribution of event water and y_w to soil water gradually decreased (100 % to 69 %). In the vertical depth, a sudden decrease in the contribution of y_w (98 % to 47 %) and event water (44 % to 11 %) in the down-slope of the soil-epikarst interface was observed. These results indicate that the infiltration of rainwater from up-slope to down-slope was altered by vertical flow to lateral flow, which was connected through the soil-epikarst interface and finally converged in the low-lying area. Contrary to expectations, the contribution of y_w at the soil-epikarst interface was lower in the shallow soil plot (43 %) than in the deep soil plot (71 %). As a supplement, moisture at the soil-epikarst interface was significantly higher in the shallow soil plot. This indicates that the renewal cycle of groundwater was slower in shallow soil-covered hillslopes not only because almost all rainwater (∼98 %) entered the soil-epikarst to redistribute as well as the higher water storage area in the soil-epikarst interface. Accordingly, the soil-epikarst structure, which is differentiated by soil depth, controls hillslope hydrological connectivity and groundwater storage capacity in the karst region of southwest China.
Analysis of runoff generation driving factors based on hydrological model and interpretable machine learning method
2022, Journal of Hydrology: Regional Studies
Xiaoqing River Basin, Shandong Province, China
Identifying the driving factors of temporal and spatial variation in runoff is key to water resource management. The traditional machine learning model lacks transparency and interpretability, which affects the wide application of machine learning in the identification of influencing factors of hydrology. Interpretable machine learning method can improve the interpretability of machine learning model. The extreme gradient boosting (XGBoost) is established based on the data generated by the calibrated Soil Water Assessment Tool (SWAT), and the XGBoost is interpreted using the Shapely additive explanations (SHAP) method to identify the impact of driving factors on runoff generation.
The results show that XGBoost can simulate the simulation ability of SWAT, and SHAP can identify the factors affecting runoff generation by interpreting XGBoost. It was found that climatic features have different effects on runoff in different sub-basins, and rainfall at high elevations (or slope) has stronger effects on runoff than that at low elevations. There is an obvious threshold effect of land use combination (or slope) on the generation of runoff, and this threshold effect is driven by high precipitation. The results of this study can provide a new method for factor analysis of runoff.
Peak discharges per unit area increase with catchment area in a high-relief mountains with permeable sedimentary bedrock
2022, Journal of Hydrology
Citation Excerpt :
We also calculated API3 and API40. Similarly, for the characterization of antecedent wetness condition from catchment information, we extracted the specific discharge at the start of each storm (qs), as previous studies have demonstrated that qs reflects the wetness conditions of the catchment (Tani, 1997; Tromp-van Meerveld and McDonnell, 2006). Radar-rain gauge-analyzed precipitation (hereafter referred to as RAP), provided by the Japan Meteorological Agency, is 1-h cumulative rainfall; it is issued every 30 min and has a spatial resolution of 1 km.
To improve the accuracy of flood-flow prediction under changing climate conditions, understanding the storm-flow generation at various scales in a catchment is essential. Flood-flow prediction is conducted assuming that peak discharge per unit area (hereafter referred to as peak unit discharge) decreases with catchment area, based on observations. However, this assumption may not be applicable in steep high-relief mountain catchments with permeable bedrock, although this has not been confirmed due to the difficulty of flood-flow measurement in high-relief mountains. We obtained continuous discharge data for three nested catchments with areas of 0.58 km², 2.2 km² and 94 km² in the high-relief Chichibu Mountains, which have permeable bedrock, and tested the hypothesis that peak unit discharge and direct runoff; flow caused by and directly following a rainfall and forms the major part of the flood hydrograph excluding base flow per unit area (hereafter referred to as unit direct runoff) were greater in larger catchments. The effect of the spatial distribution of rainfall on the spatial distribution of storm runoff was evaluated based on radar-rain gauge analyzed precipitation data after verification using ground-based rainfall data. We obtained data for 67 storms, with a depth of rainfall-event ranging from 26.5 to 231.5 mm and a return period ranging from 10^-3 to 3.2 y. During significant floods, with peak unit discharge at largest catchment was greater than 3 mm/h; peak unit discharge and unit direct runoff were always larger at larger catchment. Differences in peak unit discharge and unit direct runoff among measured catchments were often several times and sometimes more than an order of magnitude, depended on the storm events. While, the catchment mean depth of rainfall-events was up to 1.26 times greater in the largest catchment, relative to the two smaller catchments, implying that spatial variations in storm rainfall have little effect on the differences in flood flow among catchments. Our study and previous research indicate that this increasing storm runoff per unit area with catchment area is characteristic of catchments with permeable bedrock, where some rainfall that infiltrates bedrock on hillslopes is transported downstream within the bedrock to create storm flow in larger catchments.
Soil thickness controls the rainfall-runoff relationship at the karst hillslope critical zone in southwest China
2022, Journal of Hydrology
Citation Excerpt :
In contrast, the surface and bedrock topography jointly control runoff generation. Similar results have been reported elsewhere, such as in the Plastic Lake hillslope in Canada (Peters et al., 1995), the Minamitani hillslope in Japan (Tani, 1997), and the hillslope in British Columbia in Canada (Hutchinson and Moore, 2000). These studies reflect the influence of soil thickness on the hydrological process from a single watershed (McDonnell et al., 1998).
Hydrological processes in the critical zone are closely related to the soil–bedrock structures. However, the effect of soil thickness on the rainfall-runoff relationship on the hillslope with complex topography remains unclear. Surface runoff, lateral subsurface runoff from soil–epikarst interface, and epikarst runoff from the epikarst–bedrock interface were monitored on two adjacent plots with deep and shallow (66.0 vs. 35.4 cm) soil thicknesses from June 2019 to December 2020 in the karst region of southwest China. During the monitoring period, surface and subsurface runoff account for 20% and 37% of the total runoff in the deep-soil plot (DSP), and 3% and 43% in the shallow-soil plot (SSP). This demonstrates that runoff from the soil-epikarst system is predominant compared to the relatively small contribution of surface runoff. In the SSP, the surface topography wetness index (TWI) was highly coupled with bedrock TWI, and the bedrock TWI had a significant negative linear relationship (p < 0.01) with subsurface runoff. Moreover, isotope hydrogen-separation results showed that subsurface and epikarst runoff were dominated by pre-event water, but a higher contribution of event water was observed in the SSP than in the DSP. These findings supported the hypothesis that rainwater could infiltrate the epikarst more easily in shallow soil slopes. Rainfall and surface runoff exhibited a linear relationship in the dry season and a non-linear relationship in the rainy season, indicating the occurrence of threshold rainfall–runoff behavior. The rainfall amount threshold for surface runoff was higher in DSP (44.7 mm) than in SSP (39.5 mm), and the corresponding variation of rainfall intensity interpretation was greater (54% vs. 38%). For subsurface runoff, the rainfall amount threshold was higher in the DSP than in the SSP (91.0 vs 79.4 mm), and the corresponding variation of soil moisture interpretation was higher (56% vs. 20%). This demonstrated that runoff can be better predicted at deeper soil hillslopes by rainfall and antecedent soil moisture. Accordingly, this study emphasizes the importance of evaluating the spatial heterogeneity of soil thickness in hydrological process research.

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Research paperRunoff generation processes estimated from hydrological observations on a steep forested hillslope with a thin soil layer

Abstract

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

Streamflow changes after killing of pine trees by the pine-wood nematode

J. Jpn. For. Soc.

Implications for modelling surface-water hydrology

Kinematic subsurface stormflow

Water Resour. Res.

An experimental investigation of runoff production in permeable soils

Water Resour. Res.

Three-dimensional, transient, saturated-unsaturated flow in a groundwater basin

Water Resour. Res.

Effects of regrowth and afforestation on streamflow on Tatsunokuchiyama experimental watershed

Macropores and hydrologic hillslope processes

Moisture and energy conditions within a sloping soil mass during drainage

J. Geophys. Res.

A study on runoff pattern and its characteristics

On the amount of plant nutrients supplied to the ground by rainwater in adjacent open plot and forests (2)

Bull. Kyoto Univ. For.

Analysis of the aquifer by electrical resistivity protecting in Tatsunokuchiyama Experimental Forest

Research paper
Runoff generation processes estimated from hydrological observations on a steep forested hillslope with a thin soil layer