Abstract
In this paper, the authors propose a new method for low-water analysis of river fl ow by means of the function of moving mean daily rainfall.
The recession curve for successive no-rain days is expressed by the multi-exponential-type recession equation (11), or fractional type (12). As the decreasing characteristics of each exponential term of Eq.(11) are nearly equal to those of Tk-days of moving mean daily rainfall γ (Tk') and each Tk is a time constant which is defined by the reciprocal of the exponential constant ck respectively, Eq.(12) can be transformed into Eq.(37) which is composed of the sum of γ(Tk'). Eq.(37) represents the response function of a linear run-off system and that of a linear tank shown in Fig. 2 (a) .
Our proposed process for analysis is as follows.
1) Recession constant b of Eq.(12) is estimated from recession curves of observed river flow rate and the exponential recession constant ck corresponding to b is calculated using Eqs.(18)-(30).
2) Time constants Tk' to ck are defined by Eq.(40) and Tk'-days of moving mean rainfall γ(Tk') are calculated using daily rainfall data.
3) After the non-linear parts of the run-off system are cut off by the supposed Fixed-Maximum-Discharge (FMD) and Fixed-Maximum-Rainfall (FMR), proportional constants ak are botained as the partial regression coefficients by multiple regressional analysis shown in Eq.(41). In this case, FMD=25 mm/d and FMR=50 mm/d were adapted as the most desirable values and values of ak are shown in Table 5.
4) The low-water flow rate of the river in the long term is estimated using Eq.(39).
The results of analysis on the Egawa River and Honmyo River are shown in Fig. 8 and Fig. 9, respectively.
