Agricultural land-use has been reduced by mainly urbanization and devastation in Japan. The objective of this study is to evaluate the impact of the decline of agricultural land-use on flood risk and material flux in hilly and mountainous watersheds using Soil Water Assessment Tool. The results indicated that increase of flood risk due to abandonment of agricultural land-use. Furthermore, the abandonment of rice paddy field on steep slope areas may have larger impacts on sediment discharges than cultivated field. Therefore, it is suggested that prevention of expansion of abandonment of rice paddy field is an important factor in the decrease of yields of sediment and nutrients.
The changes in increasing of magnitude of flood events which have been observed all over the world could be affected by the altered hydrological dynamics as a result of climate change. The changes in flood risk affects not only human and economic damages such as inundation in the urban area but also material fluxes from watersheds such as sediment and nutrients which are eventually caused eutrophication in downstream (Chang et al., 2001; Bouraoui et al., 2004). However, it is important to consider the combine effects of climate change and land use change because land use change such as urbanization affects to hydrological dynamics as well as climate change effects. The changes in flow volume, material fluxes and timing of discharge from watersheds have been confirmed as the results of the effects of land-use changes in previous studies (Mander et al., 2000; Wheater and Evans, 2009; Tu, 2009). In Japan, total area of agricultural land-use has been decreased as one of main trends of land-use changes. A reason of the land-use change is that farmers have retired from agriculture owing to the aging of the most of farmers. Even though the aged farmers have will to continue to cultivate crops, they no longer can do it on the agricultural land-use with a steep slope such as hilly and mountainous area. For this reason, agricultural land-use which locates in hilly and mountainous areas have been decreased due to the abandonment since 1990 (MAFF, 2010). The most of abandoned agricultural land-use have changed to waste lands which will be eventually changed to forest. The land-use changing may lead to increasing flood magnitude and its frequency because the waste lands are no longer maintained by farmers. Especially, decline of rice paddy may increase flood risk because it has an ability of flood mitigation (MAFF, 2010). Furthermore, it is important to assess the impact of land-use changes in upper-middle stream as well as an assessment of climate change effects in terms of flood risk because the majority of large cities are located on downstream of large rivers such as the coastal alluvial plains. Therefore, the objective of this study is to evaluate the impact of the decline in agricultural land-use on flood risk and material flux in hilly and mountainous watersheds.
Location of the study watersheds
The four watersheds, Takahashi River (2670 km
The total area of agricultural land-use.
Analysis of land use change impacts were conducted using three scenarios: (1) base scenarios with current land-use situation, (2) urbanization scenarios in alluvial plain, (3) abandonment scenarios of agriculture in mountainous area. Potential abandonment of agricultural land-use was defined by slope steepness: the threshold was set as 5 %.
Flow duration curves of each scenarios.
The model was calibrated for the period from 1995 to 1997 by SUFI2 method
using SWATCUP version 5.1.6. The model was validated for the period from
1998 to 1999. The flood risk was assessed for the period from 2000 to 2005.
The Nash-Sutcliffe index (
The Nash–Sutcliffe index values for the validation period.
Significant changes in mean flow rate were not indicated in any scenarios in the watersheds. Although an increase of mean flow rate of 1.4 % at maximum was found in urbanization scenario in the Ashida River, the changing rates were less than 1 % in the other scenarios. The results of changes in maximum daily flow rate in the scenarios were shown in Fig. 3. The largest change of maximum daily flow rate is an increase of 12.6 % that is found in the abandoned scenario of the Ashida River. The changes in the urbanization scenario of the Takahashi River and Ashida River were 5.6 and 5.5 %, respectively. However, significant changes were not indicated in any scenarios of Takahashi River and Asahi River. It was expected that effects of the urbanization scenario on increase of flood risk were larger than the abandoned scenarios because expanding of impervious area may affect to increase of flow rate directly. Against the expectation, the results suggested that the impacts of abandonment of agricultural land-use on increase of flood risk are greater than that of urbanization. The cause of the results are considered that the differences of the total area of agricultural land-use between flat area and a steep slope area are significant. The total area of the land-use on flat areas was smaller than that on steep slope areas because agricultural land-use have been affected by urbanization already. Furthermore, steepness of the slope of agricultural land-use also affects the results. Hence, the results suggest an increase of flood risk due to abandonment of agricultural land-use which locates on a steep slope area in western Japan.
Rate of change in sediment discharge.
The estimated maximum daily sediment discharge of the Ashida River, the Takahashi River, the Asahi River and Yoshii River as base scenarios are 2488, 6966, 1478 and 30 470 Mg, respectively. Significant increases of sediment discharge are found in the abandoned scenarios; the increase rates of the Ashida River, the Asahi River and the Yoshii River were estimated to be 46, 36 and 34 %, respectively (Fig. 3). While the result of the Takahashi River indicates an increasing trend of 13 %, it is smaller than the other rivers. Although the results of the urbanization scenarios indicated increasing trends in all of the watershed as well as the abandoned scenarios, the increase rates are not significant because all of the increase rates are less than 5 %. The main difference between the Takahashi River and the other is the difference of original land-use which was abandoned. The largest area of cultivated field which was abandoned is found in the Takahashi River. Hence, the abandonment of rice paddy field on steep slope areas may have larger impacts on sediment discharges than cultivated field.
The estimated mean nitrogen discharge of the Ashida River, the Takahashi
River, the Asahi River and Yoshii River as base scenarios are 391,
1154, 1000 and 2131 Mg yr
A significant increase of organic phosphorus was found in Ashida River and Asahi River as well as the trend of organic nitrogen (Fig. 5a). The largest change rate is found as an increase of 10 % in Ashida River. However, the changes of dissolved phosphorus were not large like organic phosphorus. Both of organic and dissolved phosphorus are indicated increasing trends in all of the urbanization scenarios (Fig. 5b). Increase rate of dissolved phosphorus is larger than organic phosphorus. The large increase rates of organic nitrogen and organic phosphorus are found in the abandoned scenarios of Ashida River and Asahi River. The abandoned area of rice paddy field contributes to yields of organic nitrogen and organic phosphorus. Hence it is suggested that prevention of expansion of abandonment of rice paddy field is an important factor in the decrease of yields of sediment and nutrients.
Rate of change in nitrogen discharge in different pathways.
Rate of change in phosphorus discharge in different pathways.
The objective of this study is to evaluate the impact of the decline in agricultural land-use on flood risk and material flux in hilly and mountainous watersheds. The results indicated that increase of flood risk was due to abandonment of agricultural land-use. Furthermore, the abandonment of rice paddy field on steep slope areas may have larger impacts on sediment discharges than cultivated field. In addition, the abandoned area of rice paddy field contributes to yields of nitrogen and phosphorus. Especially, increase of sediment yield could affect the occurrence of inundation inside of levee directly because it deposits on river channel or kinds of small weirs on the channel. The flood risk of a near future in western Japan may increase due to abandonment of agricultural land-use as well as climate change effects. Therefore, it is important to prevent increase of sediment yields for decreasing flood risk. Moreover, it is suggested that prevention of expansion of abandonment of rice paddy field is an important factor in the decrease of yields of sediment and nutrients. However, this study has a limitation in terms of model structure. The dynamics of rice paddy field were mimicked like real condition in this study because the current SWAT model could not express rice paddy field as previous studies pointed out (Xie and Cui, 2011; Sakaguchi et al., 2014). Hence the results on a rice paddy field are possible to be underestimated owing to the failure of water management.
This study was supported by Grant-in-Aid for JSPS Fellows Grant Number 13J07499 and Grant-in-Aid for Young Scientists (B) Grant Number 26870837.