Spatial variability of soil hydraulic conductivity and runoff generation types in a small mountainous catchment

As an important soil property,saturated hydraulic conductivity(Ks)controls many hydrological processes,such as runoff generation types,soil moisture storage and water movement.Because of the extremely harsh natural environmental conditions and soil containing a significant fraction of gravel fragments in high-elevation mountainous catchments,the measurement data of Ks and other soil properties are seriously lacking,which leads to poor understanding on its hydrological processes and water cycle.In this study,the vertical variation(0-150 cm)of Ks and other soil properties from 38 soil profiles were measured under five different land cover types(alpine barren,forest,marshy meadow,alpine shrub and alpine meadow)in a small catchment in Qilian Mountains,northwestern China.A typical characteristic of soil in mountainous areas is widespread presence of rock and gravel,and the results showed that the more rock and gravel in the soil,the higher Ks and bulk density and the lower the soil capillary porosity,field water capacity and total porosity.The Ks of the lower layer with rock and gravel(18.49±10.22 mm·min-1)was significantly higher than that of the upper layer with relatively fine textured soil(0.18±0.18 mm·min-1).The order of values of the Ks in different land cover types was alpine barren,forest,alpine shrub,marshy meadow and alpine meadow,and the values of the Ks in the alpine barren were significantly higher than those of other land covers.Most rainfall events in the research catchment had low rain intensity(<0.04 mm·min-1),and deep percolation(DP)was the dominant runoff generation type.When the rainfall intensity increased(0.11 mm·min-1),subsurface stormflow(SSF)appeared in the alpine meadow.Infiltration excess overland flow(IOF),SSF and DP existed simultaneously only when the rainfall intensity was extremely high(1.91 mm·min-1).IOF and SSF were almost never appeared in the alpine barren because of high Ks.The alpine barren was the main runoffcontributed area in the mountainous catchment because of high Ks and low water-holding capacity,and the alpine shrub and meadow showed more ecological functions such as natural water storage and replenishment pool than contribution of runoff.

The runoff characteristics and harmonic analysis of the soil moisture dynamics in Robinia pseudoacacia stand

Robinia pseudoacacia stands act as a typical ecological protection forest in hilly semi-arid area of China. Two fields of surface runoff were separately set up in R. pseudoacacia stand and its clearcut area in the western Liaoning Province (1850-12225 E, 4024-4234 N) for measuring the characteristics of runoff and sediment as well as soil moisture dynamics. Contractive analysis of the two land types showed that there existed a significant difference in volumes of runoff and sediment between the sites of R. pseudoacacia stand and its clearcut area. The runoff volume and sediment volume in clearcut area were much bigger than those in R. pseudoacacia stand, with an increase amount of 40%-177% for runoff and 180%-400% for sediment. Hydrograph of surface runoff of typical rainfall showed that the peak value of runoff in R. pseudoacacia stand was decreased by 1.0-2.5?0-3m3s-1 compared with that in its clearcut area, and the occurring time of peak value of runoff in R. pseudoacacia stand was 10-20 min later than that in its clearcut area. Harmonic analysis of soil moisture dynamics indicated that the soil moisture in R. pseudoacacia stand was 2.3 % higher than that in clearcut area, and the soil moisture both in R. pseudoacacia stand and its clearcut area could be divided into dry season and humid season and varied periodically with annual rainfall precipitation. It was concluded that R. pseudoacacia stand plays a very important role in storing water, increasing soil moisture, and reducing surface runoff and soil erosion.

Impacts of Land-use and Land-cover Changes on River Runoff in Yellow River Basin for Period of 1956–2012

River runoff is affected by many factors, including long-term effects such as climate change that alter rainfall-runoff relationships, and short-term effects related to human intervention(e.g., dam construction, land-use and land-cover change(LUCC)). Discharge from the Yellow River system has been modified in numerous ways over the past century, not only as a result of increased demands for water from agriculture and industry, but also due to hydrological disturbance from LUCC, climate change and the construction of dams. The combined effect of these disturbances may have led to water shortages. Considering that there has been little change in long-term precipitation, dramatic decreases in water discharge may be attributed mainly to human activities, such as water usage, water transportation and dam construction. LUCC may also affect water availability, but the relative contribution of LUCC to changing discharge is unclear. In this study, the impact of LUCC on natural discharge(not including anthropogenic usage) is quantified using an attribution approach based on satellite land cover and discharge data. A retention parameter is used to relate LUCC to changes in discharge. We find that LUCC is the primary factor, and more dominant than climate change, in driving the reduction in discharge during 1956–2012, especially from the mid-1980 s to the end-1990 s. The ratio of each land class to total basin area changed significantly over the study period. Forestland and cropland increased by about 0.58% and 1.41%, respectively, and unused land decreased by 1.16%. Together, these variations resulted in changes in the retention parameter, and runoff generation showed a significant decrease after the mid-1980 s. Our findings highlight the importance of LUCC to runoff generation at the basin scale, and improve our understanding of the influence of LUCC on basin-scale hydrology.

Slope runoff study in situ using rainfall simulator in mountainous area of North China

Simulated rainfall is a valid tool to examine the runoff generation on the slope.13 simulated rainfall experiments with different rainfall intensities and durations are completed in a 5 m ×10 m experimental plot in mountainous area of North China.Simultaneously,rainfall,surface runoff,soil-layer flow,mantel-layer flow and soil moisture are monitored respectively.From the results,it is found that the hydrographs in all layers have the characteristics of rapid rise and fall.The recessions of surface flow and soil-layer flow are much faster than that of mantel-layer flow.Surface flow,the main contributor,makes up more than 60% of the total runoff in the study area.It even exceeds 90% in the cases of high intensity rainfall events.Runoff coefficient(ratio of total runoff to rainfall amount) is mainly influenced by rainfall amount,rainfall intensity and antecedent soil moisture,and the relationship can be well expressed by a multiple linear regression function α = 0.002P + 0.182i + 4.88Wa-0.821.The relation between the rainfall intensity and the lag time of three flows(surface runoff,soil-layer flow and mantel-layer flow) is shown to be exponential.Then,the result also shows that the recession constant is 0.75 for surface runoff,is 0.94 for soil-layer and mantel-layer flow in this area.In this study area,the dominant infiltration excess runoff is simulated by Horton model.About 0.10 mm/min percolation is observed under the condition of different rainfall intensities,therefore the value is regarded as the steady infiltration rate of the study area.

Effects of a check dam system on the runoff generation and concentration processes of a catchment on the Loess Plateau

As an important soil and water conservation engineering measure,more than 100,000 check dams are constructed across the Loess Plateau;these dams play a vital role in reducing floods and sediment in the region.However,the effects of check dams on hydrologic process are still unclear,particularly when they are deployed as a system for watershed soil and water management.This study examined the watershed hydrologic process modulated by the check dam system in a typical Loess Plateau catchment.By simulating scenarios with various numbers of check dams using a distributed physically-based hydro-logical model,the effects of the number of check dams on runoff generation and concentration were analyzed for the study catchment.The results showed that the presence of check dams reduced the peak discharge and the flood volume and extended the flood duration;the reduction effect on peak discharge was most significant among the three factors.The system of check dams substantially decreased the runoff coefficient,and the runoff coefficient reduction rate was greater for rainstorms with shorter return periods than for rainstorms with longer return periods.The check dams increased the capacity of the catchment regulating and storing floods and extended the average runoff concentration time in the catchment that flattened the instantaneous unit hydrograph.This study reveals the influencing mech-anism of check dam system on the watershed hydrological process under heavy rainstorm conditions and provides a theoretical basis for evaluating the effects of numerous check dams on regional hydrology and water resources on the Loess Plateau.

Xin'anjiang Nested Experimental Watershed(XAJ-NEW)for Understanding Multiscale Water Cycle:Scientific Objectives and Experimental Design

This paper presents the background,scientific objectives,experimental design,and preliminary achievements of the Xin’anjiang nested experimental watershed(XAJ-NEW),implemented in 2017 in eastern China,which has a subtropical humid monsoon climate and a total area of 2674 km2.The scientific objectives of the XAJ-NEW include building a comprehensive,multiscale,and nested hydrometeorological monitoring and experimental program,strengthening the observation of the water cycle,discovering the spatiotemporal scaling effects of hydrological processes,and revealing the mechanisms controlling runoff generation and partitioning in a typical humid,hilly area.After two years of operation,preliminary results indicated scale-dependent variability in key hydrometeorological processes and variables such as precipitation,runoff,groundwater,and soil moisture.The effects of canopy interception and runoff partitioning between the surface and subsurface were also identified.Continuous operation of this program can further reveal the mechanisms controlling runoff generation and partitioning,discover the spatiotemporal scaling effects of hydrological processes,and understand the impacts of climate change on hydrological processes.These findings provide new insights into understanding multi scale hydrological processes and their responses to meteorological forcings,improving model parameterization schemes,and enhancing weather and climate forecast skills.

Field experimental study on the effect of thawed depth of frozen alpine meadow soil on rill erosion by snowmelt waterflow

Soil erosion by snow or ice melt waterflow is an important type of soil erosion in many high-altitude and high-latitude regions and is further aggravated by climate warming.The snowmelt waterflow erosion process is affected by soil freeze-thaws and is highly dynamically variable.In this study,a methodology was developed to conduct in situ field experiments to investigate the effects of the thawed depth of the frozen soil profile on snowmelt waterflow erosion.The method was implemented on an alpine meadow soil slope at an altitude of 3700 m on the northeastern Tibetan Plateau.The erosion experiments involved five thawed soil depths of 0,10,30(35),50,and 80(100)mm under two snowmelt waterflow rates(3 and 5 L/min).When the topsoil was fully frozen or shallow-thawed(≤10 mm),its hydrothermal and structural properties caused a significant lag in the initiation of runoff and delayed soil erosion in the initial stage.The runoff and sediment concentration curves for fully frozen and shallow-thawed soil showed two-stage patterns characteristic of a sediment supply limited in the early stage and subject to hydrodynamic-controlled processes in the later stage.However,this effect did not exist where the thawed soil depth was greater than 30 mm.The deep-thawed cases(≥30 mm)showed normal hydrograph and sedigraph patterns similar to those of the unfrozen soil.The findings of this study are important for understanding the erosion rates of partially thawed soil and for improving erosion simulations in cold regions.

Hydrological processes of glacier and snow melting and runoff in the Urumqi River source region, eastern Tianshan Mountains, China

Hydrological processes were compared, with and without the influence of precipita- tion on discharge, to identify the differences between glacierized and non-glacierized catchments in the Urumqi River source region, on the northern slope of the eastern Tianshan Mountains, during the melting season （May-September） in 2011. The study was based on hydrological data observed at 10-min intervals, meteorological data observed at 15-min intervals, and glacier melting and snow observations from the Empty Cirque, Zongkong, and Urumqi Glacier No.1 gauging stations. The results indicated that the discharge differed markedly among the three gauging stations. The daily discharge was more than the nightly discharge at the Glacier No.1 gauging station, which contrasted with the patterns observed at the Zongkong and Empty Cirque gauging stations. There was a clear daily variation in the discharge at the three gauging stations, with differences in the magnitude and duration of the peak discharge. When precipitation was not considered, the time-lags between the maximum discharge and the highest temperature were 1-3 h, 10-16 h, and 5-11 h at the Glacier No.l, Empty Cirque, and Zongkong gauging stations, respectively. When precipitation was taken into consideration, the corresponding time-lags were 0-1 h, 13 h, and 6-7 h, respectively. Therefore, the duration from the generation of discharge to confluence was the shortest in the glacierized catchment and the longest in the catchment where was mainly covered by snow. It was also shown that the hydrological process from the generation of discharge to confluence shortened when precipitation was considered. The factors influencing changes in the discharge among the three gauging stations were different. For Glacier No.1 station, the discharge was mainly controlled by heat conditions in the glacierized region, and the discharge displayed an accelerated growth when the temperature exceeded 5℃ in the melt season. It was found that the englacial and subglacial drainage channel of Glacier No.1 had become simpler during the past 20 years. Its weaker retardance and storage of glacier melting water resulted in rapid discharge confluence. It was also shown that the discharge curve and the time-lag between the maximum discharge and the highest temperature could be used to reveal the evolution of the drainage system and the process of glacier and snow melting at different levels of glacier coverage.

Responses of Runoff and Soil Erosion to Vegetation Removal and Tillage on Steep Lands

Land use and land cover change is a key driver of environmental change. To investigate the runoff and erosion responses to frequent land use change on the steep lands in the Three Gorges area, China, a rainfall simulation experiment was conducted in plots randomly selected at a Sloping Land Conversion Program site with three soil surface conditions: existing vegetation cover, vegetation removal, and freshly hoed. Simulated rainfall was applied at intensities of 60 (low), 90 (medium), and 120 mm h 1 (high) in each plot. The results indicated that vegetation removal and hoeing significantly changed runoff generation. The proportion of subsurface runoff in the total runoff decreased from 30.3% to 6.2% after vegetation removal. In the hoed plots, the subsurface runoff comprised 29.1% of the total runoff under low-intensity rainfall simulation and the proportion rapidly decreased with increasing rainfall intensity. Vegetation removal and tillage also significantly increased soil erosion. The average soil erosion rates from the vegetation removal and hoed plots were 3.0 and 10.2 times larger than that in the existing vegetation cover plots, respectively. These identified that both the runoff generation mechanism and soil erosion changed as a consequence of altering land use on steep lands. Thus, conservation practices with maximum vegetation cover and minimum tillage should be used to reduce surface runoff and soil erosion on steep lands.