Interflow

Abstract: The principle that most geomorphic work is accomplished by relatively frequent events of moderate intensity requires modification for application to stream channels in certain climatic and physiographic settings. Small drainage basins in regions of highly variable flood magnitudes appear to have a high potential for catastrophic response. Flash-flood potential for small basins can be regionally mapped by computing the standard deviation of the logarithms of the annual flood peaks. Highly right-skewed flood-frequency distributions indicate that a high potential exists in certain arid regions of the southwestern United States and in the seasonal subtropical-to-steppe climate region of central Texas. High-magnitude flood response is also promoted by physiographic factors, such as hillslope morphology, soils, rock type, and drainage density. The relative importance of overland flow, which produces intense flood peaks, versus interflow and ground-water flow, which produce more uniform streamflow, appears to integrate both the climatic and the physiographic influences on the potential for catastrophic floods. Another factor in realizing the climatic-hydrologic potential for catastrophic stream-channel response is the resistance of the channel itself to scour. Small limestone streams in central Texas show significant channel modification only during the rare high-magnitude floods characteristic of that region. This is mainly because of the high response threshold required to scour bouldery alluvium and dense valley-bottom vegetation. Effects of especially intense floods on such streams include the following: entrainment of jointed bed rock and boulders as much as 3 m in diameter, uprooting of trees that usually bind coarse-grained point bars, macroturbulent transport of boulders even over divides into adjacent drainages, local scour of chutes on meander bends, and passive boulder deposition on other preflood valley-bottom surfaces.

Abstract: A modified concept of hydrological response units (HRUs) for regional modelling of river basins using the PRMS/ MMS model is presented. The HRUs are delineated by geographical information system (GIS) analysis from physiographic basin properties such as topography, soils, geology, rainfall and land use using a thorough hydrological systems analysis. The HRUs, once classified by GIS analysis, preserve the three-dimensional heterogeneity of the drainage basin. The River Brol basin (A = 216km 2 ), Rheinisches Schiefergebirge, Germany was selected to apply the concept. In total, 23 HRUs were delineated and tested with the PRMS/MMS model using a 20-year hydrometeorological daily database. The hydrological systems analysis revealed that interflow is the dominant flow process through the basin's slopes and the major contribution to groundwater recharge and river runoff. This was accounted for by parameterizing the HRUs in the model control file to drain their surplus water not used for satisfying the demand of evapotranspiration to a common conceptual subsurface storage. This storage was simulated by interflow drainage to the groundwater aquifer in the valley floor, which in turn drained to the channel network. The PRMS/MMS model simulated the observed daily discharge very well and the fit was described by a daily correlation coefficient of r = 0.91. The NASIM and HSPF models using different means to represent the basin's physiographic heterogeneity were applied to the Brol basin as well, but did not achieve this correlation. The HRU concept was found to be a reliable method for regional hydrological basin modelling and allows spatial up- and downscaling. Future research on this concept will focus on incorporating the variable precipitation distribution into the classification of HRUs and on the hydrodynamic routing of the modelled discharge. Additionally, satellite imagery must be used for classifying land use in macroscale drainage basins.

Abstract: Nitrate losses through leaching were studied in a crop field in the central Sichuan Basin, southwestern China, during 2003 to 2006. Nitrate accumulations and losses via leaching were measured using two methods: a simulation with sampled soil profiles and field monitoring in situ at a 7% slope. The results showed that NO3 was accumulated in the dry season and leached in the rainy season. No spatial dependence of soil NO3 distribution could be distinguished along the hillslope due to complex water flow. The soil interflow was the major driver of NO3 leaching losses from the experimental plots. The average annual discharge of the interflows was 148 mm, accounting for 63% of total runoff in the rainy season in 2003 to 2006. The interflow water contained high NO3―N concentrations. The highest NO3―N concentration and loss flux were detected in storm runoff after a long drought in 2006. The NO3―N leaching through interflow showed clear annual and seasonal patterns, and largely occurred in the stages from stamen elongation to maturity of maize (Zea mays L.) with a flux of 2.2 g m―2, about 69% of the total annual loss flux. Annual estimated losses of NO3―N through interflow ranged from 20 to 53, with an average of 36 kg ha―1 yr―1. This study indicates that croplands on hillslopes with regosols are vulnerable to NO3 leaching. The interflow discharge monitoring method together with effluent NO3 content analysis used in this study proved to be useful and effective for quantifying NO3 leaching losses at the field scale in hillslope areas.