Modelling soil moisture and assessing its impacts on water sharing and crop yield for the Wadi Laba spate irrigation system, Eritrea

In the Wadi Laba spate irrigation system, sorghum and maize crops complete their growth period (September to April) mainly based on the soil moisture stored during the flood season from 15 June to 15 August. For the past 100 years, the farmers have strived to divert the unpredictable and destructive floods and irrigate their fields three to four times (irrigation gift is 50 cm each time) so as to harvest 4.5 t ha-1 yr-1 of sorghum or sorghum and maize. Simultaneously, to realize fair water sharing, the farmers introduced two water rights and rules: (1) a field is entitled to a second, third or fourth turn only after all other fields have received one, two or three turns respectively; (2) upstream fields have a priority right to the medium floods (50 m3 s-1), the midstream and downstream fields to the moderately large and large (50-200 m3 s-1), and very large (200-265 m3 s-1) floods respectively. These water rights and rules were regularly observed in the indigenous systems when farmers relied on each other for the timely maintenance of the earthen/brushwood structures that frequently failed, but are being violated following the introduction of the concrete structures in 2000. The Soil Water Accounting Model (SWAM) that has been developed as part of the study on which this paper is based revealed that when a field receives two, three or four turns, the soil moisture at the onset of the planting season remains the same, namely 67.5, 72 and 77.5 cm if the field gets its last turn by 15 July, 30 July and 15 August respectively. These results were validated with the more complex Soil Water Atmosphere Plant Model (SWAP). Even the 67.5 cm water depth (with some 120 mm contribution from rainfall), sufficiently supports the said yield. Unlike in the indigenous system, when the fields usually received a third turn by the end of the flood season, the concrete structures of the modern system have made it possible for the upstream farmers to irrigate three or even four times by July. Thus, to maintain fairness of water sharing, the water rights on the size of floods would have to be modified to: regardless of the flood size, if upstream fields are irrigated twice or three times by 15-30 July, the subsequent floods would have to be conveyed to the midstream/downstream field