Water scarcity is a serious problem worldwide, which heightens the need to understand watershed dynamics and their impact on water quantity. The study examined water demand using the WEAP (Water Evaluation and Planning) model in the Arror watershed in Kenya. The primary sources of data included remotely sensed data and socio-economic data. The secondary data included climate, river discharge and soil data. Field surveys and questionnaires were used to collect socio-economic data. From the findings, the total annual water allocated (supply) for agriculture, domestic and livestock in the watershed was 10,333,441 m3, with the highest annual consumer being agriculture in the lower part of the catchment at 7,154,457 m3 for the reference scenario (1986–2012). The total mean annual demand for the same period was 10,461,123 m3 and thus a mean annual unmet demand of 127,682 m3. The highest mean monthly unmet water demand was that of agriculture in the lower part of the catchment in January (90,200 m3). Management practices that would enhance the sustainable management of water resources include construction of a reservoir and enforcement of minimum environmental flows maintenance in the river and these are recommended for the Arror watershed.