One of the significant components of the hydrological cycle is evapotranspiration. Monthly meteorological parameters of 35 years from 19 meteorological stations across the Northern Region of Nigeria (NRN) were obtained and utilized for the calibration of Hargreaves–Samani (HS) model by comparing between potential evapotranspiration (ETo) values estimated from the original HS and the Penman–Monteith (FAO-56 PM) models. The calibrated HS equation was assessed using trend patterns and some statistical indices. The average value of root mean square error (RMSE) and the mean absolute error (MAE) decreased by 37.1 and 40%, respectively, after the calibration of the model. Also, the correlation coefficients (R) of stations that had values > 0.8 increased from 6 to 11 and the minimum R value increased by 12% above that of the original HS equation. The trend and spatial map of the statistical tests conducted also indicate better performance in most climatic regions after calibration. The precision of the HS equation improved significantly after calibration for semi-arid, humid, and sub-humid regions. However, few stations in the semi-arid, humid, and sub-humid regions did not show drastic improvement due to the peculiarity of the location and high variations in the wind speed and relative humidity parameters.
Investigating the trend in meteorological and agricultural drought is important in water resources management. Drought affects agriculture, water resources and the ecosystems of Nigeria. This study investigated the trend in meteorological and agricultural drought in the guinea savannah, sudan savannah and sahel savannah agro ecological zones in Nigeria by using SPI (standardized precipitation index), SPEI (standardized precipitation and evapotranspiration index) and SSI (standardized soil moisture index) indices. SPI considers only precipitation, SPEI considers both precipitation and potential evapotranspiration to determine drought and SSI considers soil moisture to evaluate soil moisture deficit for both short and long period. Precipitation data (1981-2015) was obtained from Nigerian Meteorogical Agency (NIMET) and soil moisture data (1981-2015) for 3 weather stations (1 from each zones) was obtained from the Climate Prediction Center (CPC). The result showed that; (1) Trend analysis revealed that the hypothesis of no trend was rejected in the three agro-ecological zones studied; (2) There was an observed significant drying trend at 95% significant level; (3) Over the three zones, the drought indicators showed significant trend; and (4) Comparison analysis of the 3 drought indices used in this study shows that SPI and SPEI are more consistent in the three zones showing relative correlation compared to SSI.
Drought can generally be defined as the extreme persistence of precipitation deficit over a region for a specific period. Eight study locations were picked from the Sudano-Sahelian agro-ecological zones of Nigeria (Bauchi, Bida. Kaduna, Kano, Maiduguri. Sokoto, Nguru, and Katsina) from 1981 to 2015. The Standardized Precipitation Index (SPI), Standardized Precipitation Evaporation Index- Thornthwaite (SPEI.T), Standardized Precipitation Evaporation Index-Hargreaves (SPEI-H) and Standardized Precipitation Evaporation Index-penman (SPEI-P) were used as the primary indicators of meteorological and agricultural droughts. The correlation coefficient shows an increasing correlation among the indices with increasing time scale, with SPI and SPEI-H having the highest correlation. The regression analysis shows a monotonic increasing relationship between indices while SPI vs SPEI-H has the highest correlation coefficient. The number of drought occurrences captured by the indices also increases with increasing time scale with SPEI-P detecting the highest number of drought events. All the drought indices reflect the historical drought periods between 1982-1989, 1992-2002, and 2008-2011. SPI, SPEI-P, and SPEI-H detected similar duration and intensity for the historical drought between 1982 and 1989 while SPEI-P showed the highest intensity and duration for the historical droughts between 1992 and 2002 and between 2008 and 2011.Analytic Hierarchy Process (AHP) evaluated that SPEI-P was more robust and sophisticated, SPI and SPEI-P had the same score for tractability while SPEI-H being the least tractable, and SPI had the highest for transparency and extendibility.
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