Investigating the Available Water for Irrigation in the Tana Basin, Kenya

The objective of this research paper was to establish water resources availability for agricultural production in Tana Basin. Frequency analysis techniques are used here to determine flood and low flow magnitudes required in the design of hydraulic structures affected by high and low flows. Generally, 80% probability of exceedance is recommended so that there is sufficient water, 4 out of 5 years. The 80% probable or available flow for irrigation purposes (flow exceeded in 4 out of 5 years) was obtained from analysis of river flows for RGS 4G01 using standard probability methods. The base flow or 30% of Q95 probable flow (also referred to as the environmental flow) was also computed. The available flow for irrigation is calculated from Q80 (also called the natural flow, as it is exceeded 4 out of 5 times in a year) and the environmental flow (Q95). Values of Q80, Q90 and Q95 are shown in table 2 under section 4. The difference between the environmental and natural flow gives the available water for irrigation. Kenyan regulations require that 30% of the Q95 base flow be considered as the environmental flow. The available water for irrigation is therefore calculated from equation 15. This formula is also called the water balance calculation for irrigation. Low flow analysis data has been presented for exceedance probabilities of 80 and 90% (Q80 and Q95). The Q80 flow is normally considered as the reliable flow for irrigation purposes. For Tana at Garissa, 4G01, the reliable flow varies from 35.7 cumecs in March to 122.6 cumecs in November. The available water is maximum in April-May when the demand is lowest.