ABSTRACT:The Niger River is a lifeline for humans and biodiversity in Africa's Sahelian region. Owing to climate change and population increase over the past three decades, the inhabitants of the Sahel have witnessed that their ecosystem is under threat, and a significant reduction of its resources has occurred, particularly in the Inner Niger Delta. In the recent years, because of the Malian government's policy of developing large-scale irrigation, a sustained rate of expansion of around 5,000 ha of new reclamation area has been seen each year in Office du Niger located upstream of the Inner Delta. The expansion of annual rice double cropping and sugarcane cultivation constitute the main factors for the reduction of the water resources downstream in this Delta. This paper analyzes the hydrological impacts of this large-scale irrigation and crop pattern on the floodplain of the Inner Delta during high-flow and low-flow periods. The study results indicated that despite the rainfall and recent increases in upstream flows of the Niger River, the flooded area size has reduced. We could confirm that the annual average water-withdrawal proportion barely changed during the high-flow period but changed significantly during the low-flow period when the size of the paddy rice and sugarcane area increased. Therefore, changing the crop pattern toward vegetables instead of paddy rice and sugarcane during the low-flow period might be a solution.

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Estimation of reference evapotranspiration (ET0) with the Food and Agricultural Organisation (FAO) Penman-Monteith model requires temperature, relative humidity, solar radiation, and wind speed data. The lack of availability of the complete data set at some meteorological stations is a severe restriction for the application of this model. To overcome this problem, ET0 can be calculated using alternative data, which can be obtained via procedures proposed in FAO paper No.56. To confirm the validity of reference evapotranspiration calculated using alternative data (ET0(Alt)), the root mean square error (RMSE) needs to be estimated; lower values of RMSE indicate better validity. However, RMSE does not explain the mechanism of error formation in a model equation; explaining the mechanism of error formation is useful for future model improvement. Furthermore, for calculating RMSE, ET0 calculations based on both complete and alternative data are necessary. An error propagation approach was introduced in this study both for estimating RMSE and for explaining the mechanism of error formation by using data from a 30-year period from 48 different locations in Japan. From the results, RMSE was confirmed to be proportional to the value produced by the error propagation approach (ΔET0). Therefore, the error propagation approach is applicable to estimating the RMSE of ET0(Alt) in the range of 12%. Furthermore, the error of ET0(Alt) is not only related to the variables’ uncertainty but also to the combination of the variables in the equation.

In order to prove a reduction function concerning flood damage according to land use, we have to compare evaluation indices, such as flood concentration times. These can be derived from the calculation of the outflows from two basins where rainfall, geology, geographical conditions, etc. are the same and only the land use is different. However, it is very difficult to obtain comparable data from large basins under the above mentioned conditions. An elaborate observation for a long period is required to obtain such data. In this paper, we aim to establish a calculation process that offers consideration factors from a viewpoint that is different from a conventional research where elaborate observation is essential. Therefore, first we use easily obtainable data of many basins, such as the information on the Internet, or data from official documents or the like, which are difficult to deal with in hydrology. Then, we obtain several evaluation indices, which are coefficients close to the flood concentration times in this paper, on each basin in which various kinds of land uses are mixed. We then perform the statistical analysis with a land use ratio. In order to use various data on several basins, we evaluate the difference in the land use ratio in the basin by using of a length of a riverside instead of an area in the basin. Moreover, since a river-mouth is at the end of the riverside and a water stage observation point is not at the river-mouth, an error arises. Although there are several problems in accuracy as mentioned above, the evaluation of the reduction function concerning the flood damage according to land use will be feasible, automatically and efficiently, if the calculation process is established.