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Lowland irrigated rice is the predominant crop produced in the Senegal River Valley characterized by very low annual rainfall, high temperatures, and low relative humidity. The Senegal River is shared by Senegal, Mali, Mauritania, and Guinea, and serves as the main source of irrigation water for the adopted double rice cropping system. Developing appropriate resource management strategies might be the key factor for the sustainability of rice production in the region. This study aims to estimate rice seasonal evapotranspiration (ETa), irrigation water requirement, and to develop rice growth stage specific crop coefficients (Kc) to improve rice water productivity. Field experiments were conducted during the hot and dry seasons in 2014 and 2015 at the AfricaRice research station at Fanaye in Senegal. Irrigation water inputs were monitored and actual crop evapotranspiration was derived using the water balance method. Daily reference evapotranspiration (ETo) was estimated using the Penman-Monteith equation and the weather variables were collected at the site by an automated weather station. The results showed that the ETo during the hot and dry season from February 15 to June 30 varied from 4.5 to 9.9 mm and from 3.7 to 10.8 mm in 2014 and 2015, respectively, and averaged 6.8 mm d in 2014 and 6.6 mm d in 2015. The seasonal irrigation water amount for the transplanted rice was 1110 mm in 2014 and 1095 mm in 2015. Rice daily ETa varied from 4.7 to 10.5 mm in 2014 and from 4.4 to 10.5 mm in 2015 and averaged 8.17 mm in 2014 and 8.14 mm in 2015. Rice seasonal ETa was 841.5 mm in 2014 and 855.4 mm in 2015. The derived rice Kc values varied from 0.77 to 1.51 in 2014 and 0.85 to 1.50 in 2015. Rice Kc values averaged 1.01, 1.31, and 1.12 for the crop development, mid-season and late season growth stages, respectively. The Kc values developed in this study could be used for water management under rice production during the hot and dry season in the Senegal River Valley. Ac ce pt ed p ap er Introduction Rice is the principal staple food widely consumed and a veritable source of calories for humans. Rice is primarily produced under flooded conditions, but it is not an aquatic plant. Rice water use needs to be investigated under climate change conditions due to decreasing trends in available fresh water for agriculture coupled with increasing world population. Rice is an important staple food crop in Senegal and he main crop grown across the Senegal River valley and Delta and 100% irrigated. Under the semiarid climate conditions in the Senegal River Valley, accurate estimation of rice actual evapotranspiration is fundamental for sustainable water management for improving rice water productivity. Irrigated lowland rice water use differs with agroecosystems, climate, and management practices. Rice gross water use was 2300 mm in Tanzania with very low water productivity of 0.3 kg m (Mdemu et al., 2004). Jehangir et al. (2004) found rice water requirement that varied from 1200 to 1600 mm in the Pakistan sub-tropical semiarid conditions. Tuong et al. (2005) reported irrigation water inputs from 400 to over 2000 mm with median values in the range of 1300 to 1500 mm. Hargreaves et al. (2006) reported rice irrigation water requirement of 1788 and 2030 mm for the wet and dry seasons in the Senegal River Basin, while recent studies reported irrigation requirements of 1110 to 1300 mm (de Vries et al., 2010) and 863 to 1198 mm (Djaman et al., 2016a). A clear gap exists between irrigation water use and water requirements for rice production. Rice water use under flooded irrigation is well documented in many Asian countries and other locations across the globe. Seasonal rice actual evapotranspiration (ETa) varied between 400 and 700 mm in the Philippines (Tabbal et al., 2002) and 540 and 730 mm in India (Chahal et al., 2007). Rice ETa range of 750 850 mm was reported by Aguilar and Borjas (2005) and Moratiel and Martinez-Cob (2013) under sprinkler irrigation in Spain. In Italy rice water use was within the range of 700 to 800 mm (Spanu et al., 2009). Large variation in rice seasonal irrigation requirement is observed among rice hubs and at the same Ac ce pt ed p ap er rice production site with regard to interannual variation in climate. Rice seasonal irrigation water requirements varied from 962 to 1114 mm in Taiwan (Kuo et al., 2006), from 383 to 1148 mm in the sub basin of Niger River in Benin (Bouraima et al., 2015). Under the semiarid climate in the Senegal River Basin Djaman et al. (2016a) reported rice water requirements values that varied from 863 to 1198 mm while Terjung et al. (1984) reported rice irrigation water requirements over 1000 mm in northwest China and 500 mm in southcentral China. Irrigation water requirements of most paddy fields is high considering water losses by seepage as well as low efficiency of delivered water by most irrigation canals by not being sealed. Rice ET is often times estimated by the two-step approach, multiplying reference ET by growth stage specific crop coefficients (Kc) (Jensen, 1968; Jensen et al., 1990; Allen et al. 1998). This method is an alternative to the direct measurements of crop actual evapotranspiration through lysimeters, Bowen ratio energy balance systems, eddy correlation systems, flux profile techniques, and surface renewal (Hatfield, 1990; Rudnick and Irmak, 2014). Rice growth specific Kc values have been developed and reported for the major rice hubs to improve water and nutrient management in the paddy fields. Allen et al. (1998) recommended rice Kc values of 1.05, 1.20, and 0.90 to 0.60 for the crop development, midseason and late season growth periods under continuous flooding irrigation conditions. Kc values of 0.92, 1.06, and 1.03 were reported for sprinkler irrigated rice under semiarid climate in Spain (Moratiel and Martinez-Cob, 2013). While Arif et al. (2012) reported low rice Kc value of 0.70 for the initial stage, they reported higher midseason, and late season Kc values of 1.24, and 1.22, respectively, under intermittent irrigation and 1.21, and 1.10 under continuous flooding in Indonesia. Similarly, high Kc values of 1.15-1.58, 1.44-1.75, 1.901.96, 1.59-1.82, and 1.0-1.41 for the tillering, panicle initiation, flowering, physiological maturity, and harvesting were reported in India by Choudhury and Singh (2016) reported Ac ce pt ed p ap er transplanted flooded rice. In California, Montazar et al. (2017) derived paddy rice Kc values of 1.10, 1.00, and 0.80 for the initial, midseason and late season stages. Although Kc values are developed and proposed for several locations under different climate as aforementioned, a few data and information is available on the sub-Saharan Africa rice hubs. Growth stage specific Kc values are influenced by irrigation regime, management practices, local climate, soil types, and other environmental factors (Allen et al., 1998; Djaman and Irmak, 2013), and therefore, it is important to develop growth stage specific Kc values under local management practices for accurate estimation of crop water use. The objectives of the present study were to estimate rice actual evapotranspiration and to derive rice crop coefficients using the water balance equation for the period of 2014-2015 under semiarid climate at Fanaye in the Senegal River Valley, Senegal. Materials and methods Site description and data collection The study was conducted at Fanaye in the Senegal River Valley (SRV) (Senegal, West Africa) during the hot and dry seasons in 2014 and 2015. The experimental site is located on the Africa Rice Center (AfricaRice) research station at Fanaye (16 32’ N, 15 11’W). Climate at the site is typically Sahelian with 9 month dry period from October to June and short wet season from July to September. Temperatures are high from March to July. Typically, rice production takes place twice a year, from February to June in the hot dry season and from August to December in the wet season. The soil type at the station is characterized as a eutric vertisol with high clay content (45 to 65%) and a percolation rate of 2.0 mm d (Samba, 1998; Haefele, 2001). Climatic variables at the site were collected between February and June in 2014 and 2015. Daily average wind speed, maximum and minimum air temperature, maximum and minimum Ac ce pt ed p ap er relative humidity, incoming solar radiation, and precipitation were measured over a wellwatered grass surface using an automated weather station (CimAGRO) installed at the experimental field station. The weather station area is 64 m (8 m over 8 m). Cynodon dactylon (L.) generally called Bermuda grass was planted and irrigation by basin irrigation. Water is conducted to the plot through PVC pipe and diverted in the middle of the plot. No standing water was allowed within the grass. The frequency of irrigation was twice a week or as needed. Grass was regularly cut and had average height of 12 cm as recommended. Grass plot was kept weed free with no pest or rodent damage. The automatic agro-weather station CimAGRO is a compact system designed with Institut national de la recherche agronomique, (INRA) partnership and is equipped with extremely reliable and stable sensors that are interchangeable without programming (plug and play connections) and suitable under difficult weather conditions. All variables were sampled every 60 seconds and recorded on daily basis. Crop management Rice seed of the variety Sahel 108 was pre-germinated on February 15 2014 and 2015 and transplanted on March 7 and 9 in 2014 and 2015, respectively, at the rate of 25 hills m. The plot area was 1250 m (25 m X 50 m). Fertilizers in the form of urea, ammonium phosphate, and potassium chloride were applied to achieve 120, 26, and 50 kg ha of nitrogen (N), phosphorous (P), and potassium (K), respectively. Fertilizer was split and applied as follows: 50% N, 100% P, and 100% K were broadcast 14 days after transplanting; the remaining N fertilizer was s

Actual evapotranspiration and crop coefficients of irrigated lowland rice (Oryza sativa L.) under semiarid climate