Seasonal rainfall variability and aptness of Geographical-Information- Systems (GIS) interpolation techniques in the arid regions of Embu county, Kenya
Ngetich, F. K.
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This study sought to characterize inter/intra seasonal rainfall variability, drought probabilities and assess the efficacy of geo-statistical interpolation techniques for spatio-temporal reconstruction of rainfall data in arid areas of Embu County; Kenya. Gaps in rainfall data from two stations of Embu and Machang’a were filled using multiple imputations. Cumulative Departure Index (CDI), Rainfall Anomaly Index (RAI) and Coefficients-of-Variance (CV) and probabilistic statistics were utilized in the analyses. Data reconstruction utilized ArcGIS environmental tool combined with the digital elevation model (DEM) to generate average spatial rainfall and maps using various interpolation techniques. The efficacy of interpolation techniques was assessed using root mean square errors (RMSE), mean absolute errors (MAE) statistics plus gauged-data for validation. Rainfall homogeneity was accepted at 99% probabilities. Probabilities of rainfall exceeding cropping threshold were 50% (506.8mm) at Embu and 30% (523.7mm) at Machang’a during Long-Rains (LRs) and Short-Rains (SRs) respectively. High variability was observed in rainfall amounts (CV=0.41 and 0.36) during LRs and (CV=0.56 and 0.38) during SRs in Machang’a and Embu respectively. Daily rainfall distribution depths were highly skewed; small proportion of rainy days supplying a high proportion of rainfall. Variabilities in rainy days were CV=0.26 and 0.08 (LRs) and CV=0.88 and 0.27 (SRs) in respective stations above. High variability were observably in March (onset) (CV=0.98 and 0.61) and October (onset) (CV=0.80 and 0.66) respectively. Dry-spell probabilities within growing months were high (81%) and (60%) in Machang’a and Embu respectively. Kriging technique was identified as the most appropriate Geo-statistical and deterministic interpolation techniques that can be used in the region. To optimize yield in the area, use of soil-water conservation and supplementary irrigation, crop selection and timely accurate rainfall forecasting should be prioritized. Key words: Cumulative-Departure-Index, GIS, Interpolation, Kriging, Rainfall-Anomaly-Index, Rainfall-variability