Journal of Hydrologic Engineering

Knowledge of soil saturated hydraulic conductivity (Ks) and its spatial variation is important to characterize hydrological processes. The objective was to quantify three important variability components, i.e., measurement technique, spatial arrangement in sampling, and differing landscape features (sloped surfaces and gullies), to map Ks distribution in eroded watersheds, such as the Chinese Loess Plateau. Classic and geostatistics were employed to explore the effects of measurement method, sampling scale, and landscape position on the Ks and its spatial variation. In a small watershed with an area of $0.2{\mathrm{km}}^2$, three techniques, including the soil core (SC), tension infiltration (TI), and Guelph permeameter (GP) methods, were employed to determine the surface Ks at 124 positions on the sloped landscape (minimum sampling space of approximately 10 m). Furthermore, the SC method was applied at 204 positions along four 100-m transects (minimum sampling space of 2 m), and 200 positions along four 10-m transects (minimum sampling space of 0.2 m) on the sloped landscape and 45 positions in the gully areas to determine the surface Ks. The results showed that no significant differences of mean logarithmic value of Ks (${\mathrm{Log}}_{10}\mathrm{Ks}$) existed for SC and GP, and the spatial patterns behaved the same for TI and GP. The spatial dependence decreased with a decrease in sampling extent. Structured variability was not observed at the 10-m transect scale. When the minimum sampling space changed from 10 to 2 m, the nugget variance decreased, whereas the structured variance, sill variance, and spatial dependence increased. When minimum sampling space changed from 2 to 0.2 m, the changes in spatial pattern of Ks were negligible, implying that a minimum sampling space of 2 m is needed to capture the spatial pattern of Ks. Ks in the gully areas was significantly less than on the sloped positions. The spatial pattern of Ks changed when the gully positions were included. Gully areas, therefore, should be considered for characterizing the spatial pattern of Ks for watersheds in the Chinese Loess Plateau.

Biological soil crusts (biocrusts) cover up to 60–70% of the soil surface in grasslands rehabilitated since the Grain for Green project was implemented in the hilly Loess Plateau region in 1999, which exerted significant impacts on runoff and soil loss from revegetated grasslands. In the study, field plots were used to investigate runoff and soil loss in sites of a 4- and a 13-year revegetated grassland, with each exhibiting an early and a later successional biocrust, respectively. The objectives of the study were to (1) examine the role of biocrusts on runoff and soil loss during their early and later successional stages in a semiarid region under water erosion, (2) determine the influence of biocrusts on soil antiscourability with different runoff intensities, and (3) isolate the effects of biocrust patches and vascular plant canopies on runoff and soil loss from revegetated grasslands. Treatments used in both sites included (1) retaining biocrusts and plant canopies intact (CP), (2) retaining biocrusts without plant canopies (CNP), (3) retaining plant canopies without biocrusts (PNC), and (4) removing both biocrusts and plant canopies (NCP). The simulated scouring water flux was designed as 7.8, 12.0, and $16.0\mathrm{L}·{\min }^{-1}$ to reflect local rainfall conditions. The results indicated that the runoff yield was increased by biocrust patches in their well-development stage. Runoff was increased by 15.1% when plant canopies were retained and 16.0% when plant canopies were removed in the 13-year revegetated grassland with the $12.0\mathrm{L}·{\min }^{-1}$ scouring water flux. Compared with biocrust patches, plant canopies reduced runoff by 11.3% (with biocrusts) and 8.4% (biocrusts was removed) with the same scouring water flux. No significant difference was found in runoff yield with respect to the four treatments in the 4-year revegetated grassland. In contrast, 92% of the sediments were reduced for the formation of biocrusts in their early successional stage (cyanobacteria-dominated biocrusts) in the 4-year revegetated grassland with respect to CNP compared with NCP at the $12.0\mathrm{L}/\min$ scouring intensity. No sediment was generated on either CP or CNP treatments in grassland revegetated for 13 years (moss-dominated biocrusts) with the same intensity of simulated runoff. Compared with biocrusts, plant canopies had a limited influence on soil loss. This amounted to reductions of 45 and 10% in soil loss for grasslands that revegetated for 4 and 13 years, respectively. The results of the study suggest that biocrusts play an important role in soil loss control from water erosion in semiarid regions, although there was a potential increase in runoff yield.

A field experiment was carried out to research the effect of soil temperature distribution on root water uptake in soil water simulation. Soil temperature distribution patterns under border irrigation and surface drip irrigation were researched. The root water uptake model was modified based on the effect of soil temperature on root water uptake. Results showed soil temperature profile distribution was greatly influenced by irrigation method. The range of temperature was larger under border irrigation, with the temperature being 3–6°C higher in 0–20 cm depth than in 20–100 cm depth. Except for the top layer under surface drip irrigation, mean soil temperature showed the trend of exponential decay throughout the soil profile. The relationship between temperature and water uptake rate was expressed in exponential function. With the modification of the root water uptake model as affected by temperature profile distribution, the value of the root mean square error between the simulated and observed soil water decreased from approximately 0.04 to 0.02 in the top layer under border irrigation, but showed no obvious difference under surface drip irrigation. When soil temperature differed greatly in the top layer from the deep layer, the root water uptake model considering soil temperature could improve the precision of soil water simulation. The results indicated that the modified root water uptake model could be used to simulate soil water dynamics.

The hydrological cycle in a catchment is sensitive to climate and land-use changes. The authors conducted a case study to validate the performance of the soil and water assessment tool (SWAT) and its applicability as a simulator of runoff and sediment transport processes at the mesoscale scale in arid and semiarid areas. SWAT is used to simulate runoff and sediment changes caused by human activities in a typical watershed, the Jihe Watershed ($1,019{\mathrm{km}}^2$), in the Loess Plateau of northwestern China. A marked increase in temperature was observed over the analysis period. The investigation was conducted using 47 years of historical rainfall/runoff data and sedimentary records from 1962–2008. The data from 1962–1981 was used for calibration and that from 1982–2008 for validation. The results showed that the Nash-Sutcliffe model efficiency coefficient was approximately 0.7, the relative error was less than 15%, and the coefficient of determination was greater than 0.7, both for annual flow and sediment yield in the calibration period. These findings indicate that the SWAT model was able to simulate runoff and sediment yield satisfactorily; however, it exhibited better performance for the calibration period than for the validation period. Similarly, simulations of monthly flow and sediment were better for the calibration period. The simulated and observed values agree well with trend changes. Uncertainty analysis indicates that digital elevation model resolutions and watershed subdivisions imposed little influence on annual flow, but notable effects were observed with respect to annual sediment yield.

In the Loess Plateau of China, mosaic-vegetation restoration by converting cropland into fallow (to regenerate natural vegetations) and perennials performs well in soil erosion control. However, soil desiccation caused by the planted perennials threatens the sustainability of vegetation restoration. Understanding of soil water distribution and balance in monovegetation systems at hillslope scale is crucial for building a constructive mosaic-vegetation restoration pattern in the Northern Loess Plateau. The objectives in this study were to investigate effects of monovegetation restoration types on soil water distribution and balance and discuss the possible implications of the monovegetation hydrological properties for mosaic-vegetation pattern establishment at hillslope scale. In 2004, the authors chose a one-piece waste hillslope with uniform slope of 12° and established four monovegetation plots subjected to shrub, grass, fallow, and cropland. Shrub, grass, and fallow are the typical vegetation restoration types, whereas cropland presents the traditional land use. Soil water content profiles, down to 400–600 cm depth, along the hillslope were measured with a neutron moisture meter from May–October in 2004, 2008, and 2009. Results showed that the rainfall infiltration depth was approximately 100 cm for shrub and grass but exceeded 300 cm for fallow and cropland. Six growth years later, shrub, grass, and fallow depleted more soil water than cropland, in the amount of 288, 313, and 62 mm, respectively. Soil water depletion in shrub and grass resulted in dried soil layers at the depth of 100–260 cm and 100–360 cm. Water balance results indicated that soil water deficit occurred in June during the rain season. The authors observed that downhill-accumulation of soil water storage, down to 400 cm depth, existed for fallow and cropland in 2004 and 2009. However, 6 growth years of shrub and grass substantially weakened such soil water downhill-accumulation tendency. This fact alone suggests that a mosaic vegetation system of planting shrub/grass downhill and setting fallow uphill would be appropriate from a standpoint of maintaining the sustainable development of vegetation restoration in the study area. Further experiments should be performed to develop the mosaic-vegetation patterns meeting the interests of both erosion control and sustainability of vegetation restoration.

Climatic variability and human activities are the two primary factors that affect basin hydrology, and thus quantification of their effects is of great importance for water resources management and sustainable development at a catchment scale. In this study, the writers investigated the long-term trends and abrupt changes in hydroclimatic variables, including precipitation, potential evapotranspiration (PET), and runoff, from 1957–2000 in the Hutuo River Basin by the nonparametric Mann-Kendall test and the precipitation-runoff double cumulative curve method. A two-parameter hydrological model and linear regression method were employed to separate and quantify the effects of climatic variability and human activities on runoff. The results are the following: (1) significant downward trends for annual precipitation and annual runoff were detected by the Mann-Kendall test at a 99% confidence level, (2) a change in the gradient of precipitation-runoff double cumulative curves and an abrupt change in runoff series can both be found in 1979, indicating that the relationship between precipitation and runoff has changed; as a result, the annual runoff from 1957–2000 can be divided into two periods termed the baseline (1957–1979) and human-induced (1980–2000) periods, and (3) the climate variability was the primary cause for the decrease in annual runoff from the baseline to the human-induced period, despite certain effects of human activities on the change with respect to annual runoff.

Field capacity is one of the most commonly used soil parameters in irrigation systems and agriculture. Methods for determining field capacity are either time, pressure, or model based. The numerical approach to estimate field capacity using a model-based definition is found to be effective in its ability to predict field capacities; however, this modeling-based approach is difficult to run for some soil as the numerical model requires an extremely small time increment close to zero and some oscillations are produced while the top part of the soil column approaches near saturation. In this study, an approach using the Richards’ equation in conjunction with the modified van Genuchten–Mualem (MMVG) model to simulate variably saturated flow in estimating field capacity is developed. The MMVG model is found to be useful to avoid the numerical oscillations, and there is less effect from the negligible flux associated with the estimate of field capacity. For estimating the real-world soils’ field capacity, a combined soil data set was developed from two wide-ranging databases. A new neural-network pedotransfer function (PTF) was developed to match data set values and objected values calculated in the MMVG model. Using this numerical approach together with the new PTF also showed better agreement with the filed measurements than benchmarked pressure heads and the model-based method with the original MVG models. The dependency of field capacity on soil texture was shown by the contour diagrams. The potential application of this approach was shown in the distribution of field capacity in China.

Hydraulic lift is the process by which some deep-rooting plants take in water from deep soil layers and exude that water into the upper, drier soil layers through the root system. It is favorable for plants that transport the water and may be an important water source for neighboring plants. Hydraulic lift increases the stability of plantation communities and improves the growth of plants on semiarid sandy soil. The objectives of this study were to examine the hydraulic lift ability of sandy plants and define the characteristic of this process in the Horqin Sandy Land. Growth boxes were used to cultivate examined plants, isolating the upper and the lower layers with Vaseline petroleum jelly. The soil water supply of topsoil in growth boxes was controlled during the examination period, and the hydraulic lift was confirmed by measuring the variations in soil water content in the topsoil in the growth boxes. Hydraulic lift was investigated in 21 native, deep-rooting plants in the Horqin Sandy Land. The hydraulic lift process may be common to deep-rooting psammophytes, and it mostly occurs from 2400 to 0600 hrs. The quantity of hydraulic lift water among the 19 species varied. The increment of soil water content lifted by each gram of root biomass within 24 h were between 4.86 and $325.62\mathrm{g}·{\mathrm{g}}^{-1}$, with an average of $72.90\mathrm{g}·{\mathrm{g}}^{-1}$. The species Artemisia wudanica, Artemisia gmelinii, Thermopsis lanceolata, and Bassia dasyphylla have the strongest hydraulic lift abilities among all examined species. No distinctive correlation was observed between the degree of drought in the topsoil and the total water lifted by the process. The hydraulic lift of psammophytes can be used to improve the water content of shallow soil, which may be significant in vegetation establishment on semiarid sandy land.

In the mountainous area of China, the rock fragments in top soils are often removed by farmers because of their side effects on tillage. To understand hydrological processes and to assess the risk of soil erosion in purple soil areas with sloping farmlands, this study investigated the effects of rock fragment cover on surface runoff, infiltration, subsurface runoff, and erosion under field conditions using a portable rainfall simulator. Experimental twin plots (two subplots, 1 m wide by 2 m long, 32% slope) with different rock fragment covers ranging from 0 to 42% were exposed to four rainfall intensities: $45.5\pm 1.9\mathrm{mm}/\mathrm{h}$ (I1), $60.3\pm 3.6\mathrm{mm}/\mathrm{h}$ (I2), $92.0\pm 4.1\mathrm{mm}/\mathrm{h}$ (I3), and $123.9\pm 9.4\mathrm{mm}/\mathrm{h}$ (I4). Surface runoff, subsurface runoff, soil moisture, and sediment were measured. The results show that the hydrological response was influenced by the rock fragment cover and rainfall intensity. The rate of surface runoff in bare soils was affected by the rainfall intensity and increased with increasing rainfall intensity. The values of the surface runoff rates in bare soils were lower than $70\mathrm{mm}/\mathrm{h}$ under the I1, I2, and I3 rainfall intensities. As the rock fragment cover increased, the surface runoff rate decreased while the infiltration rate, the subsurface runoff rate and the deep percolation coefficient increased. On the other hand, when the surface runoff rates in the bare soils were higher than $70\mathrm{mm}/\mathrm{h}$ (in rainfall of I4), there were no significant differences in these hydrological variables among soils with different rock fragment covers. The differences in hydrological variables among soils with varying rock fragment covers decreased with increasing rainfall intensity. The rock fragment cover determined the erosive response. As the rock fragment cover increased, the sediment concentration decreased. The presence of surface rock fragments significantly reduces soil erosion, and the relationship between the soil erosion rate and rock fragment cover can be expressed by an exponential function with a high degree of reliability for different rainfall intensities. However, the effectiveness of the rock fragment cover in reducing soil erosion decreased with increasing rainfall intensity.

Water problems and water crises are of widespread concern around the world against the background of climate change and intense human activity. As the only source of drinking water in Beijing being confronted with severe water shortages, the Chaobai River Basin is extraordinarily important. Land use, being a common and variable influence on streamflow, has been studied widely, whereas spatiotemporal differences and the complex mechanism of land use’s effect on runoff are difficult to explain. On the basis of a series of data about land use and runoff depth in the Chaobai River Basin, the sensitivity of land-use change to streamflow was explored using the principle of elasticity. Results of $t$-tests and $F$-tests showed that the difference in stream flow between the two river basins is not explained by precipitation; thus, land-use change is the main cause of stream flow variation. The influence of forests on annual runoff depth was significant and increased gradually from 1978 to 2008, whereas the effect of bare land has been insignificant in all time periods. The effect of land use on seasonal and monthly runoff depth showed significant variation. On the whole, the effects of land-use change on streamflow in summer were obvious, especially in August. It is therefore concluded that forest changes are the main factors in streamflow variation in summer.

Water flushing was applied to different land use plots on Loess slopes to examine the effect of land use changes on flow hydraulics and the transport of soil surface solutes in erosion. The runoff and movement of sediment and soil solutes were analyzed in relation to land use and scouring flow. Flow experiments were conducted with five land use treatments: abandoned land (Salsola ruthenica), alfalfa land (Medicago sativa), corn land, scrub land (Caragana intermedia), and bare land. The results show that at the same scouring time, the cumulative sediment yields with different land use types are: bare land > corn land > Caragana intermedia land > abandoned land > alfalfa land. The unit sediment loads are similar to this modulus of the cumulative sediment yields. The pre-experimental water contents of the soil profile exerted a greater effect on the content of soil moisture and its distribution on the slope than that of the antecedent solute contents in the field experiment conditions for different land use types. The land use types also affected the surface transport of soil solute. The bromide concentrations in runoff were in the order of: bare land > corn land > alfalfa land > Caragana intermedia land > abandoned land. The nitrate concentrations in runoff with different land use types had no obvious orderliness. However, the nitrate concentration was lineally related to the bromide concentration, as expressed by ${\mathrm{C}}_{{\mathrm{NO}}_3^{-}}=3.01{\mathrm{C}}_{{\mathrm{Br}}^{-}}+28.35$ ($R^2=0.90$).

The authors have studied the principal components of rainfall interception loss in a planted forest stand of Robinia pseudoacacia on the Loess Plateau. The purpose was to provide new information about the applicability of the original Gash analytical model to a new geographic location and to one of the primary species being used in the region’s reforestation program. The authors estimated forest structure parameters, including the mean evaporation rate, canopy storage capacity at saturation, free throughfall coefficient, rainfall fraction diverted to the trunks, and trunk storage capacity by using the intercepts and slopes obtained from regression analyses of the measured interception loss, throughfall, and stemflow versus gross rainfall. The interception and components of interception loss for trees in a Robinia pseudoacacia forest located on a south-facing slope were calculated using Gash’s analytical model. The total estimated interception loss during the period of observation was 10.8% greater than that calculated on the basis of measurements of the gross rainfall, throughfall, and stemflow. The good agreement between the estimated and measured values indicates that Gash’s analytical model is suitable for estimating interception loss in forests on the Loess Plateau of China.