Recently, a lot of researchers have been interested in `scale issues' in the research area of hydrology. 'Scale issues' include various questions to be solved, including lots of issues regarding runoff process. For example, the relationships between some hydrological processes which control runoff process and catchment scale have been studied. Recently, some researchers have tried to clarify the runoff system using mean annual flood (MAF) per unit area and coefficient of variation (CV) of annual flood.In this study, analysis of runoff process by CV and MAF was carried out. First, CV/MAF versus catchment area were plotted for 285 catchments in Japan. Secondly, Eagleson's method was used in order to calculate MAF and CV. The aim of this study is to develop model for numerical analysis of runoff, which will be combined to analysis by the data of 285 catchments. The model used in this study could estimate MAF qualitatively, but further consideration is necessary in regard to CV.

Abstract:A series of wind tunnel experiments were conducted to investigate the effect of surface roughness distribution on evaporation processes. In particular, the relationship between non-homogeneous roughness distribution parameters and effective roughness parameters, which are often required to estimate the evaporation rate over a heterogeneous area, were examined closely. A number of sand ridges having various distributions were placed perpendicular to the main wind direction on a 3 m long lysimeter (1 m in width and 0Ð6 m in depth) filled with fine sand. The groundwater table in the lysimeter was maintained at a constant level ranging from 0Ð1 to 0Ð55 m below the surface. The lysimeter was fitted to the wind tunnel, in which incoming airflow was controlled to have a constant velocity, temperature, and humidity. An internal boundary layer was formed on the sand surface and the mean profiles of wind velocity, humidity, and temperature were observed within this layer. In total, 99 profiles over 33 different surfaces were obtained. The roughness parameters for each surface were evaluated by fitting the mean profile equations to the observed profiles. The following effects of roughness distribution on evaporation and the effective roughness parameters were found.(1) An increase in standard deviation of the roughness distribution, which is the variation in obstacle intervals, caused an increase in the evaporation rate when the average roughness spacing was constant and relatively large. (2) The interrelationships among effective roughness z m , z v , and z h can be described by the same function that describes that of homogeneous surfaces. (3) Evaporation from rough surfaces was more sensitive to the depth of groundwater than that from the smooth surface within our experimental conditions, because the rough surfaces allow soil water over a broader range of depth to reach the surface. The first two results suggest that the effective roughness parameters are a function of the average and perturbation of roughness distribution on the surface and that the effective z v and z h values can be estimated from effective z m value.

No abstract

No abstract

Abstract:Scaling can be a powerful solution for predictions in ungauged basins (PUB). Since becoming a principal scaling tool, the theory concerning the index flood method has been criticized because it requires some scaling conditions that are satisfied in few river basins. In that method, precipitation and flood discharge variability play key roles. Consequently, the coefficient of variation (CV) of annual flood peaks came to be considered after the 1990s. In this paper, we have attempted to clarify true CV characteristics of flood discharges.Using numerical simulations, we attempt to reproduce the empirical characteristics of an increase in CV with increasing catchment area for a small basin (i.e. less than 30-100 km 2 ) and decrease in CV with increasing catchment area for a large basin. First, as a preliminary test, we developed a simple model, which is on the basis of the unit hydrograph. Results obtained using this simple model show that the CV of annual flood peaks shows a constant-increasing-constant pattern, but this model cannot reproduce the empirical characteristics. Secondly, we used an idealized channel network to find the cause of decreasing CV. However, results show that the ability to reproduce the empirical characteristics is unrelated to the presence or absence of a network. This was confirmed using a distributed rainfall-runoff (DRR) model comprising a channel network. The most important cause is decreased CV of rainfall intensity with increasing catchment area.Furthermore, increasing CV of the peak discharge response (PDR) function increases the CV of annual flood peaks. However, the CV of PDR of a partial duration series does not affect the CV of annual flood peaks for smaller basins (0Ð1 km 2 in our simulation condition); hence CV is constant for small basins.