Estimation of Potential Sediment Yield by integrating USLE with GIS -A case study at Tenryu Watershed in Central Japan- Kazutoshi HOSHIKAWA, Masashi KAWABATA, Madhusudan B. SHRESTHA and Jun SUZUKI Faculty of Agriculture, Shinshu University 8304 Minamiminowa, Kamiina, Nagano, 399-4598, Japan, [email protected] Abstract: The Japanese archipelago has high potentiality in producing huge sediments due to its topographical and hydro-meteorological conditions. Of all, Tenryu basin, having its headwater sources in the highest mountain ranges, the Japanese Alps in Central Japan, is one of the highest sediment yield river. Frequent fluctuation of channel beds and deposition of huge sediments in the reservoirs have been a severe problem. In order to control flood and manage river system, an integrated management of entire watershed is considered to be a desirable strategy; therefore, estimation of potential sediment yield in a large watershed, has been an important issue. This study was attempted to estimate the sediment yield at eight sub-watersheds: Yokokawa, Katagiri, Matsukawa, Shintoyone, Minowa, Miwa, Koshibu and Misakubo watersheds of the Tenryu basin using USLE model incorporated with GIS techniques, and validated the estimated sediment yield with the actual sedimentation measured from their corresponding reservoirs. From the study, it was found that the variation of observed annual sedimentation at reservoirs has showed similar trend with the estimated USLE values within the studied watershed. This outcome suggests the reliability and adaptability of the USLE method that is integrated with GIS in estimating annual potential sediment yield in the large watershed having complex topography and geology. Keywords: USLE, GIS, Sediment production, Tenryu River sub watershed 1. Introduction Most of the rivers flowing in the mountainous region of Central Japan are rapid, and their flow rates are seasonally highly variable. Huge sediment yields due to heavy rains subjected to frequent seasonal rain fronts and typhoons have caused several sediment disasters in the past. These disastrous sediments have adverse impacts on: river environment; sandy shores around the estuarine region; and efficiency in trapping sediment in the dams/reservoirs. Furthermore, in recent years due to the global climate changes, risks of sediment disasters subjected by the frequent occurrences of instantaneous intensive rainfall have been increasing; as a result, change in river dynamics has been occurring frequently which further unfavorably affecting the river environment and ecosystem. Hence, interests and concerns towards the disaster prevention and mitigation strategies for the entire watershed have been growing. The study area Tenryu (TR) basin (5,090 km2), derived from its river Tenryu, having its headwater sources at the Japanese Alps of the Central Japan, has the precipitous terrain and fragile geology causing high sediment yields in the region. Flowing through many towns and cities of three prefectures―Nagano, Shizuoka and Aichi, Tenryu River has many important aspects from the social, economical and cultural point of views. Although the problems of high sediment yield are prevailing across the basin, a decisive method for estimating sediment yield, especially in the macroscales has yet to be established. Generally, while performing sediment yield estimation in the rivers, two apparent major issues evolve. They are: 1) estimation of sediment yield due to sheet-rill-gully erosions, and mass movement ―landslides/landslips, slope failures, etc. within the watershed; and 2) a pragmatic method for determining the sediment dynamics (sediment transportation and deposition) ―fluvial geomorphological activities within the watershed. As these issues are closely and mutually inter-related to each other and have various temporal and spatial changes, they have made the process more complicated for estimating sediment yield in a watershed. There have been numerous models (both empirical and process-based) developed in the past to predict soil loss at a field or catchment’s level. The applications of these models require various parameters which express the small spatial and temporal conditions of a field. Since the phenomena are complex and depend on many parameters, the collections of many suitable parameters are difficult and calibration of models is complicate. Therefore, an evaluation of potentially suitable models that can be used with readily available input data is an important step in using them for practical applications [1, 2]. However, the task for enlarging the USLE applications from small plots to larger watersheds and determining the correct values for the major equation factors is still challenging [3]. In this study, USLE [4] was used to estimate the potential sediment-yield at eight sub-watersheds of the TR basin for a period of 21 years (1983-2003). Although this method is originally developed for the conservation of agricultural land in a small field, its use has been extended to watershed with extensive coverage and other complicated land uses. The advantage of using USLE model is that it has been widely tested over many years and the validity and limitations of this model is already known [5]. Computed sediment yields were compared and evaluated with the deposited sediments at the reservoirs locating at outlets of the corresponding sub-watersheds. As sediment yield from upland areas is a large source of sediment that ultimately transported into the reservoirs; thus, the reservoir sedimentation can be used to determine sediment yield from the watershed [6]. Furthermore, developments on Geographic Information Systems (GIS) and Remote Sensing (RS) techniques and consolidation of geological and topological data incorporated with GIS system have also brought simplicity and high precision in determining sediment yield of a watershed. The aim of this study is mainly of twofold: 1) to compare estimated values with actual deposited sediment; and 2) to investigate the applicability of the USLE method at macroscales level in a large mountainous watershed having complex topographical features. 2. Method and materials 2.1. Study area The study area, Tenryu basin, sandwiched between Southern and Central Japanese Alps was chosen for the study because of its significant sediment yield characteristics. As shown in Fig. 1, Tenryu River (213 km) originates from the Yatsugadake Ranges and flows through the Inadani (Ina valley), a whip-shaped valley along the Median Tectonic Line (MTL), an active tectonic line and a major structural and lithologic boundary, extended between Southern Alps (Akaishi Ranges) and Central Alps (Kiso Ranges). The Central Alps are mostly underlain by late Cretaceous granitic rocks and the Southern Alps by Mesozoic sedimentary rocks. Sediment production by weathering is predominant in granitic catchments and shallow and deep-seated landslides are in shale, metamorphic catchments (sedimentary rocks). Due to having temperate climate (annual average temp. of 10) and monsoon climate (precipitation of about 2000 mm yr-1), denudation processes are very active in the TR basin [7]. The combination of disastrous geology and, intense and large amount of rainfalls have brought about several catastrophic sediment disasters giving its name as “Abare(Savage) Tenryu”. After the devastating and catastrophic sediment disasters in 1961, 1968 and 1970, several dams/reservoirs: Koshibu dam, Matsukawa dam, Katagiri dam, Yokokawa dam, Minowa dam and Shintoyone dam were constructed for flood control, hydroelectric power generation, irrigation, urban water supply, recreation and mitigation sediment disasters. Eight sub-basins: Yokokawa watershed (YKW); Katagiri watershed (KGW); Matsukawa watershed (MKW); Shintoyone watershed (STW); Minowa watershed (MNW); Miwa watershed (MW); Koshibu watershed (KSW); and Misakubo watershed (MSW) of the TR basin were selected for the study (Table 1). The first four sub-basins: YKW, KGW, MKW and STW are located at the right bank (west to MTL) and the rest are located in the left bank (east to MTL) of Tenryu River. Most of the sub-basins have comparatively small basin areas from 15 to 60 km2, high basin relief and steep slopes characterizing rapid sediment transportation in the basins. Table 1 Characteristics of drainage basins and their impoundments Watershed Capacity of Average Min. altitude Max. altitude Basin relief Sub watersheds Area impoundments altitude (m) (m) (m) (km2) (103m3) (m) Yokokawa Watershed 38.8 186 912 2135 1223 1449 (YKW) Katagiri watershed 15.1 184 934 2242 1308 1653 (KGW) Matsukawa watershed 60 740 699 2296 1597 1512 (MKW) Shintoyone watershed 136.3 5350 425 1364 939 827 (STW) Minowa watershed 38.2 950 825 1518 693 1129 (MNW) Miwa watershed 311 2995 519 2960 2441 1149 (MW) Koshibu watershed 288 5800 758 2886 2128 1824 (KSW) Misakubo watershed 57 3000 465 1991 1526 1134 (MSW) 2.2. Universal Soil Loss Equation (USLE) The Universal Soil Loss Eq. (1) has the following model structure4: A = R K L S C P. (1) Where, A is computed site soil loss (t ha-1 yr-1), R is a rainfall erosivity factor (MJ mm ha-1 h-1 yr-1), K is a soil erodibility factor (t h MJ-1 mm-1), L is a slope length factor that is often combined with S, a slope steepness factor, to yield a unitless terrain factor (LS), C is a unitless vegetation cover factor and P is a dimensionless erosion control practice factor. The methods used for determining five USLE factors are summarized below. 1) Rainfall Erosivity Factor (R) The erosivity factor R