A study of losses from field channels under arid region conditions

Abstract

In arid regions, water losses from unlined small field channels are usually high due to seepage and evaporation from open surfaces. These losses are often neglected by many project planners and engineers. A theoretical analysis has been developed to modify the equation usually used to determine the water losses based on the ponding method, where the channel longitudinal slope was considered in the analysis. A field investigation has been carried out in sandy soil to determine and evaluate the water losses for three different types of channels. They are: earthen-uncompacted channel, compacted channel bed and channel lined by jute mats coated with bitumen emulsion on both faces. The last two cases are relatively low-cost, need less skillful labour than lining by cement and are more suitable for temporary field channels. Manning's coefficient was determined for each case. The results show that the process of compating the channel bed reduced the rate of seepage by a considerable value and that lining of field channels by prefabricated bitumen jute mats caused a significant reduction in the seepage rate. The results also show that the evaporation from open surfaces caused a considerable loss and should be considered when studying water losses from irrigation channels in arid regions.

Abbreviations A = channel surface area ⋅ A_c = channel cross section-area ⋅ b = channel bed width ⋅ d = observed difference in Class-A pan ⋅ h₁ = original water depth in the canal ⋅ h₂ = canal water depth after a certain time ⋅ k = constant ⋅ L = canal length ⋅ n = Manning's coefficient ⋅ p = average wetted perimeter ⋅ q_evap = evaporation losses rate ⋅ q_s = canal seepage losses rate ⋅ q_t = total losses from the canal ⋅ Q = canal discharge ⋅ R = penetration resistance ⋅ R_h = hydraulic radius of the channel ⋅ s = channel longitudinal slope ⋅ t = time ⋅ V_e = volume of water lost by evaporation ⋅ v_m = channel mean velocity ⋅ v_s = channel surface velocity ⋅ V_s = volume of water lost by seepage ⋅ V_t = total volume of water lost ⋅ w = water surface width of the canal ⋅ y₁ = downstream water depth at time zero ⋅ y₂ = downstream water depth after a time t ⋅ z = canal side slope