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Runoff and sediment loads from river basin are largely affected by the interplay of climate variability and human activities within the basin. However, distinguishing the impacts of climate variability and human activities would vastly improve our knowledge of water resources, climate variability and climate adaptation, and watershed management. We propose a new and simple method to determine the impact of human activities within paired datasets under the same or similar weather conditions (SWC). These weather conditions cover one or more meteorological elements such as precipitation, temperature, or evaporation. If there are two or more periods with similar weather conditions but different runoff, the relative runoff and sediment load changes can be considered a consequence of human-induced land surface changes. This study will report on the application of this new method, using the Yan River Basin in China as a case study. We found 10 sets PPs (paired periods) in 1 year intervals and 12 sets of PPs in inter- vals of 3 years when (1) there was a 2.0% and 1.0% difference of annual precipitation and annual ET0, respectively, (2) the relationship between monthly precipitation and ET0 of PPs was significant (P < 0.05) and, (3) there was no overlap of years for the PPs with intervals of 1 and 3 years. We found that the impact of human activities varied greatly between PPs, with the main trend of declining PPs, matched the trends evident from statistical analysis and land use and land cover (LULC) change evaluation. The method is simple and easily applicable to selected periods in most areas and could be extended when more detailed data are available. The result of this method is the impact of all human activities, allowing for further discussion on the contributions of each kind of human activity over time in determining the range which links the research results at different scales, e.g. to define the sediment delivery ratio (SDR) describing soil erosion on catchment slopes and sediment load in the river.

Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52 m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a two-dimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.

The present study suggests that many soils in the Netherlands may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. The persistence of water repellency was examined in topsoils using the water drop penetration time (WDPT) test. The severity of water repellency measured on dried soil samples, the so-called “potential” water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. Measurement of the “actual” water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The “critical soil water content”, below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies. Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm3 steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This study provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

The variability of soil water content over short distances was studied at seven experimental fields, located in southern Sweden, eastern Germany, southern France, and in the Netherlands. The soils all in use as arable land, are a sandy soil, a stony sandy soil, a loamy sand, a sandy loam, and three clay soils. Samples with a volume of 100 cm 3 were taken at close intervals in trenches at several depths to determine potential water repellency and soil water content. When dry, the topsoils of the Mellby site in Sweden (loamy sand) and the Vredepeel site in the Netherlands (sandy soil) are water repellent, whereas the soils at the other five sites are wettable, according to the water drop penetration time (WDPT) test. The variation in water content within short distances was high at most depths at all sites, with differences of 7 to 18 vol% within a horizontal distance of only centimeters to decimeters. Drier as well as wetter soil areas were visualized by contour plots of the soil water content distributions in the transects. Large differences in wetting capacity between samples taken in the topsoil and in the subsoil at several sites were assessed by measurements of the wetting rate. In many cases, subsoil samples wetted faster than topsoil samples. The severity of water repellency of sandy samples from the Vredepeel site increased remarkably at oven temperatures above 65°C. The wetting rates of dried samples from this soil were also evidently influenced by the oven temperature; samples dried at 25°C wetted immediately and reached soil water contents of 23 to 32 vol% within one hour, whereas samples dried above 85°C hardly wetted during 72 hours, as a result of the increased water repellency at higher drying temperatures.

Water repellent soils are known to inhibit water infiltration, ultimately forcing water to flow via preferential paths through the vadose zone. To study water flow and transport in a water repellent sandy soil, a bromide tracer experiment was carried out, which started in the fall, after winter wheat had been sown. Field average soil water content and bromide profiles were determined seven times during a 474 d period. A one-dimensional convection–dispersion model was used to simulate observed flow and transport. It was found that water content and bromide profiles could be successfully simulated. The main reason was that despite water repellency no distinct preferential flow occurred within this particular experimental field. Only wavy wetting fronts were observed. This was due to a uniform vegetation cover and a relatively thick A-horizon (30 cm), which contrasts with known water repellent sandy soils with thin (10 cm) top layers in which typical fingered flow patterns had previously been found.

Sealing, crusting and cracking of crusts of the soil surface has been observed in many parts of the world in areas with sandy, silty and loamy soils. Sealing and crust formation occurs under the influence of rain storm and drying weather. With prolonged drying, surface crusts might crack, leading to complex situations with respect to infiltration and runoff generation. Cracking of crusted loamy soils appears to be a general process. This study aims to measure the hydraulic properties of fully crusted and cracked-crusted areas and to evaluate the effects of these measurements on catchment discharge and soil loss in a loess region of the Netherlands, using the LISEM soil erosion model. Samples with minimum infiltration rates (fully crusted) and with maximum infiltration rates (cracked-crusted surfaces) were taken from fields with bare soil or winter wheat and their soil hydraulic functions were measured. The results of these measurements were used as input in the LISEM soil erosion model. Simulations of discharge and soil loss were done for each of these two land-uses and for two rain events. Additionally, simulated discharge and soil loss under actual recorded land-use were calculated. In all cases, soils with no surface cracks produced higher figures for discharge and soil loss than those where 10% of the surface crust was cracked. For a good interpretation of the results for soil loss, the spatial distribution of cracked-crusted areas and fully crusted areas has to be investigated in detail. To deal with cracked-crusted and fully crusted areas in simulation modelling, care has to be taken to accurately measure the soil physical functions representing the maximum and minimum infiltration rates. An assignment of these functions to calculation grids has to be made. As the LISEM model is capable of assigning different soil physical functions to each calculation grid, an improved prediction of the soil physical behaviour of the catchment can be simulated.

Rapid response tensiometers or TDR (time domain reflectometry) probes in soil profiles, or early arrival of solutes in groundwater or drainwater after a rain event, provide direct evidence of preferential flow in soils. However, little information about the amount and magnitude of preferential flow paths is obtained by such measurement methods. Here, two intensively sampled vertical trenches illustrate the effect of sample spacing and the effect of sample size on the detection of preferential flow paths. In a water-repellent sandy soil, a sample spacing of up to 22 cm over a distance of several metres is just sufficient to collect information about preferential flow paths. Using larger sample spacings, the water content distributions apparently became more horizontally stratified. Increasing the sample size by pooling pairs of adjacent 100 cm 3 soil samples over a distance of several metres, still allowed the detection of preferential flow paths. Preferential flow paths were no longer observed for larger sample sizes. Enlarging the sample size reduces the calculated standard deviation and coefficient of variation. As preferential flow paths may vary in space and time, so the optimal number of samples to detect these paths in vertical trenches may vary, indicating that sampling strategies need to be flexible in design.

Recent research has shown that irregular wetting and preferential flow may occur in heterogeneous field soils. Bare dune sands, which at first sight show the greatest homogeneity, were sampled volumetrically at nine locations in temperate and arid areas to determine the spatial distribution of soil moisture content, θ, and dry bulk density. Soil moisture patterns were irregular, even in extremely dry situations. As κ - θ functions for dune sands are very steep in the dry θ range, small differences in water content result in huge differences in hydraulic conductivity, κ. These differences, induced by soil moisture variability, may result in anisotropy of the system, and hysteresis tends to magnify this phenomenon. So, if soil moisture differences exist, they will be consolidated and even amplified during new rain events. At one site on a recently deposited dune sand, infiltrating water was able to transform a one-layer soil system into a two-layer soil system, inducing preferential flow at the layer interface. At another site, preferential flow paths appeared at the soil surface as ‘sand columns’ as a result of the displacement of the dry sand between the flow paths, caused by wind erosion. In all the dunes studied, dry bulk density patterns were irregular and tended to be slightly vertically orientated. Spatial distributions of volumetric and gravimetric soil water content showed great similarity.