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.
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.
ABSTRACT:The aim of this study is to contribute in irrigation scheduling by proposing adaptable models that are widely used for the estimation of reference evapotranspiration ( 0 ) in Herat, Afghanistan. Six well-known models, The Penman-Monteith ( 0 ), Hargreaves ( 0 ), Hamon ( 0 ), Thornthwaite ( 0 ), solar radiation based ( 0 ) and Net radiation based ( 0 ) were compared, and the pan evapotranspiration � � model was used as indicator. The pan coefficient ( ) proposed by Pereira was used to convert pan evaporation ( ) to . Results obtained showed that, the 0 values estimated by all the methods were shown to be close to those of in the second period (spring, fall and winter). However, large differences emerged in the first period (the windy summer), with the exception of 0 . This method displayed a small difference only in June and July. Pearson's correlation ( ) showed that the estimates produced by all the simpler methods were significant correlated with those of in the second period, but weakly correlated in the first period. The 0 method produced the lowest value of 1.3 mm day -1 , based on the standard error estimation ( ). The seasonally-based average difference between and 0 was smaller than that of the other methods in the first period, at 1.9 mm day -1 . The 0 estimation rate was therefore closest to . It is concluded that the methods that used wind factor are more adaptable than those not used wind factor especially in Herat, Afghanistan. The wind might be the reason of the differences between and 0 in the windy summer.
Optimal estimation of reference evapotranspiration ( 0 ) is extremely important to calculate irrigation scheduling in Afghanistan. In this study, a measured evapotranspiration from an A-class pan ( ) was selected as an index to discuss the error of 0 which was calculated using the Penman-Monteith (FAO-56PM) method in the west region of Afghanistan, which is exposed to extreme climate condition. Results obtained showed that the period from June to September was confirmed as extreme, with out-of-normal-range climatic data, for example, high temperature, low humidity, and relatively strong wind speed. While for the rest of the year, they were almost within the normal range of the climatic variables. By comparing the daily ave rage 0 and , the differences between 0 and were significantly large in the period from June to September, while this differentiation was very small outside of this period. By comparing the relationship of error with climatic variables, it was found that the relationship of the error with wind speed was strongest compared to the other three variables. The higher the wind, the larger the difference, and vice versa. Therefore, experimentally it was confirmed that this kind of error becomes larger when 0 is greater than 10 mm d −1 .
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