Supplemental irrigation potential and impact on downstream flow of Karkheh River Basin of Iran

Abstract

Supplemental irrigation of rainfed winter crops improves and stabilizes crop yield and water productivity. Although yield increases by supplemental irrigation are well established at the field level, its potential extent and impact on water resources at the basin level are less researched. This work presents a GIS-based methodology for identifying areas that are potentially suitable for supplemental irrigation and a computer routine for allocating stream flow for supplemental irrigation in different subbasins. A case study is presented for the 42 908 km<sup>2</sup> upper Karkheh River Basin (KRB) in Iran, which has 15 840 km<sup>2</sup> of rainfed crop areas. Rainfed crop areas within 1 km from the streams, with slope classes 0–5%, 0–8%, 0–12% and 0–20%, were assumed to be suitable for supplemental irrigation. Four stream flow conditions (normal, normal with environmental flow requirements, drought and drought with environmental flow) were considered for the allocation of water resources. Thirty-seven percent (5801 km<sup>2</sup>) of the rainfed croplands had slopes less than 5%. Sixty-one percent (3559 km<sup>2</sup>) of this land was suitable for supplemental irrigation, but only 22% (1278 km<sup>2</sup>) could be served with irrigation in both fall (75 mm) and spring (100 mm), under normal flow conditions. If irrigation would be allocated to all suitable land with slopes up to 20%, 2057 km<sup>2</sup> could be irrigated. This would reduce the average annual outflow of the upper KRB by 9%. If environmental flow requirements are considered, a maximum (0–20% slopes) of 1444 km<sup>2</sup> could receive supplemental irrigation. Under drought conditions a maximum of 1013 km<sup>2</sup> could be irrigated, while the outflow would again be reduced by 9%. Thus, the withdrawal of steam flow for supplemental irrigation has relatively little effect on the outflow of the upper KRB. However, if the main policy goal would be to improve rainfed areas throughout the upper KRB, options for storing surface water need to be developed

Accounting for environmental flow requirements in global water assessments

“…This approach can guide policy-makers who have to decide for EFRs values in different river basins where ecological and hydrological data are poor and it could be a starting point to implement EFRs at river basin level with "fair" ecological conditions. In future global EF assessments, it will be important to consider the inter-annual variability of flow regimes because EFRs are usually calculated on a long period average (> 20 years) and they might need to be refined for dry years (Hessari et al, 2012). Regarding the use of ecological data sets, it is worth considering the delay in ecosystem response related to flow events when calculating EFRs (Sun et al, 2008).…”

Moving beyond ‘more crop per drop’: insights from two decades of research on agricultural water productivity

“…Studies on this pathway discuss as possible interventions management practices for improving the timing of water supplies using supplemental irrigation or deficit irrigation. In dry regions, moisture availability, especially during critical periods, is frequently the most significant factor limiting agricultural production (Oweis, Hachum & Kijne, 1999;Hessari, Bruggeman, Akhoond-Ali, Oweis & Abbasi, 2012). Several longer-term studies, conducted in northern Syria, found that rainfall supplemented by irrigation increases water productivity (in terms of yield per unit of water consumed) in wheat systems by alleviating moisture stress during the most sensitive stages of crop growth period (Oweis et al 1999;Zhang & Oweis, 1999;Oweis & Hachum, 2003).…”

Beyond “More Crop per Drop”: evolving thinking on agricultural water productivity

“…In dry regions, moisture availability, especially during critical periods, is frequently the most significant factor limiting agricultural production. Research carried out through the SWIM, CA and subsequent programs explored the extent to which supplemental irrigation, often coupled with rainwater harvesting, can enhance yields as well as water productivity in arid and semi-arid regions (e.g., Oweis et al 1999;Wani et al 2009;Hessari et al 2012).…”