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Estimation of annual suspended sediment yield from a Japanese cypress (Chamaecyparis obtusa) plantation considering antecedent rainfalls

Ide, Jun'ichiro, Kume, Tomonori, Wakiyama, Yoshifumi, Higashi, Naoko, Chiwa, Masaaki, Otsuki, Kyoichi
Forest ecology and management 2009 v.257 no.9 pp. 1955-1965
Chamaecyparis obtusa, forest plantations, runoff, sediment yield, statistical models, regression analysis, coniferous forests, forested watersheds, soil erosion, hysteresis, equations, rain, history, estimation, accuracy, forest litter, understory, ground vegetation, vegetation cover, Japan
Japanese cypress (Chamaecyparis obtusa) is one of the most common plantation species in Japan. In most Japanese cypress plantations, closure of the forest canopy hinders understory growth because of insufficient management practices. Thus leaf litter barely covers the soil surface. Such plantations are vulnerable to surface erosion triggered by rainfalls, and could yield large amounts of suspended sediment (SS). However, few studies have investigated the annual SS yield. This study aimed to develop a modified model of SS yields, and to accurately estimate and characterize the annual SS yield from a Japanese cypress plantation. For this, hydrological surveys were conducted for 5 years, and stream water was sampled weekly as well as sequentially at 15-60min intervals in each of 14 rain events in a small forested watershed of Japanese cypress. The root mean square error (RMSE) and the annual SS yield estimates were obtained using three different equations: the rating curve derived from instantaneous SS concentration versus stream discharge relationship (SSC- Q equation); the rating curve derived from cumulative SS yield versus cumulative specific discharge relationship ([summation operator]SS-[summation operator]Q equation); and the multiple regression of cumulative SS yield expressed as functions of cumulative specific discharge and antecedent rainfalls ([summation operator]SS-[summation operator]Q xAR equation). The [summation operator]SS-[summation operator]Q xAR equation is a modified model considering the effects of antecedent rain history on the SS yield. The parameters in each equation were determined by a nonlinear least square method. Of the three equations, the RMSE was the highest using the SSC- Q equation. Although the [summation operator]SS-[summation operator]Q equation and the [summation operator]SS-[summation operator]Q xAR equation gave similar RMSE values, the SS yields extrapolated by the [summation operator]SS-[summation operator]Q equation would be overestimated. Thus, the 5-year average of annual SS yield estimated by the [summation operator]SS-[summation operator]Q equation was three times larger than that estimated by the [summation operator]SS-[summation operator]Q xAR equation. These results indicate that the [summation operator]SS-[summation operator]Q xAR equation more accurately estimates the annual SS yield, compared with the other two equations. The results also suggest that antecedent rain history, as well as stream discharge, should be considered for appropriately estimating the annual SS yield. The annual SS yield estimated by the [summation operator]SS-[summation operator]Q xAR equation was the largest among 25 undisturbed forested watersheds, except for two watersheds dominated by sandstone. Our results suggest that the annual SS yield is greater from a Japanese cypress plantation under current management practices than from other undisturbed forests.