Estimated Irrigation Water Requirements of Rice Using Cropwat Model in Benin’s Sub-Basin of Niger River

Understanding crop water needs in semi-arid region is essential for irrigation scheduling and water saving measures because of water supply is limited. This paper estimated the reference evapotranspiration (ETo) and the water requirement of the rice crop (Oryza sativa) in Benin’s sub-basin of Niger River (BSBNR) west Africa, using CROP WAT model. A monthly meteorological weather data recorded from1943to2010, rainfall, crop and soil data were used as inputs in Cropwat model to obtain the result. The recommended Penman-Monteith reference crop evapotranspiration (ETO) with derived crop coefficients (Kc) from the phenomenological stages of rice is used to estimate the crop water requirements (CWRs).The BSBNR has a typical West African inter-tropical climate by a unique modal rainfall pattern with two seasons:a rainy season, April to October and a dry season, November to March. The average annual rainfall is849mm uneven distributed over year. The months June to September are the wettest,90%of cumulative annual rainfall,30%of August alone is257.5mm. The average monthly maximum temperatures vary between39℃in April and30℃in August and the relative humidity varies between25%and81%. High maximum Humidity is observed from April to December.The morphology of the region is relative flat topography marked by plains, shallow riverbeds with stagnant and poorly flowing stream and rivers.The valleys within BSBNR are characterized by hydromorphic soils of silts and clays which reduce infiltration and maintain the growth of swamp vegetation. The soil water content of a silt-clay is estimated by a field capacity (FC) ranges from0.30to0.42m3.m-3, a Wilting point (WP) ranges from0.17to0.29m3.m-3and a reserve (FC WP) of0.13to0.19m3.m-3. The Surface water potential exceeds the2485billion cubic meters.An area of516ha of rice is cropping by nursery and transplanting. Oryza sativa is the one of the cropping rice in BSBNR. Diverse varieties of it have been introduced. The "ADNY11" cycle length is120days and stages period are15/35/50/20in days for Initial stage, Development stage, Midseason stage and late season respectively. The crop coefficient (Kc) for the first tenths decades are1.1and for the last decade0.95that explain they have higher evaporation of rice crop for the first decades of its cycle. Seeding rate is about30cm x10cm;25cm x15cm or20cm x20cm in rainy lowland. Sowing is made in discontinues lines within pockets but there are also sowing in continuous lines. The recommended fertilizers are:60N and40P2O5with about100kg/ha of urea. Planting is made between June15and July15in rainy season and between mid-November and early December in irrigated dry season period. The uneven distribution of rice sowing is cause of uneven distribution of harvesting date. To simplify our analyses on crop water requirement, we have been considered the June15th for rainy season cropping and November15th for the dry season cropping.The BSBNR annual reference evapotranspiration (ETo) is estimated at1967mm. Lowest ETo value,123mm, is observed in august during rainy season while highest value of210mm is observed in march during dry season. As8months out of12months have rainfall values below100mm/month, the fixed80%method on effective rainfall calculation have been used in Cropwat. It estimated the annual rainfall at849mm in which679mm of effective rainfalls.The total crop evapotranspiration (ET) is about669.5mm in rainy season while it is879.4mm for the dry season. The ET values for rainy season ranges from658.6mm to669.5mm; Effective Rain values ranges from607.6mm and636.4mm while Irrigation requirement ranges from349.4mm375.9mm. Thus ET values are higher than Effective Rain which superior to Irrigation requirement (ET> Rain> Irr.Req). The ET values for dry season ranges from879.4mm to909.4mm, Effective Rain from4.9mm to4.4mm and Irrigation requirement from1121.5mm to1156.2mm. ET values are inferior to Irrigation requirement which inferior Effective Rain (ET<Irr.Req.<Rain). Hence the irrigation requirement for dry season1121.5mm is higher than for rainy season349.4mm. Crop consumes more water in dry season than rainy season and need to be more irrigated in dry season without rainfalls almost.