Environmental flows in support of sustainable intensification of agriculture in the Letaba River Basin, South Africa

This study evaluates the socioecological consequences of the potential trade-offs between maintaining environmental flows (e-flows) and providing water for sustainable subsistence agriculture and livelihoods to the vulnerable human communities living along the lower Great Letaba River in South Africa. Implementation of e-flows is now generally recognized as an essential part of water resources management as they are designed to ensure that sufficient water is retained in a river to protect river ecosystems and all the beneficiaries of services that arise from those ecosystems. Understanding the relationship between e-flows and the use of water for small-scale agriculture is important for the management of trade-offs. The Letaba River Basin and it's tributary, the Great/Groot Letaba, are located in the eastern part of the Limpopo province in South Africa. This is one of the most important river basins in the region supporting both large-scale commercial and small-scale farmers. The river sustains many vulnerable human communities who depend on the ecosystem services provided by the river. Yet, the water resources of the Letaba River are heavily overutilized due to expanding developments, including upstream dams with associated offtakes mostly for irrigation. The findings of the study indicate that irrigation water demand from subsistence agriculture in the Great Letaba Basin amounted to around 2 million cubic meters annually with median demand not exceeding 300,000 cubic meters per month. This means that irrigation water demand from smallholder agriculture only amounts to about one-tenth of the estimated e-flow requirement. However, small-scale farmers contend with an increasing crop water gap which limits irrigated agriculture, especially during the dry season. Given the need to sustainably maintain e-flows for ecological purposes, crop water gaps are only likely to increase and compromise the sustainability of irrigated agriculture. With active upstream supplementation of river flows from dams to maintain both environmental and livelihoods-oriented river flows, the crop water gap can be fully eliminated. This supplementation is not assured due to competing uses.

本研究针对南非大莱塔巴河下游沿岸弱势人类社区，评估了维持环境流量（environmental flows, e-flows）与为其提供可持续自给农业用水及生计用水之间潜在权衡所引发的社会生态后果。目前，环境流量的实施已被广泛认定为水资源管理的必要组成部分，其设计初衷是保障河道留存充足水量，以保护河流生态系统及源自该系统的各类服务受益群体。厘清环境流量与小规模农业用水之间的关联，对权衡关系的管理具有重要意义。莱塔巴流域及其支流大/格罗特莱塔巴河（Great/Groot Letaba）地处南非林波波省东部，是该区域兼具大型商业农业与小规模农户农业的核心流域之一。该流域维系着大量依赖河流提供的生态系统服务的弱势人类社区。然而，受上游水坝建设及配套取水设施（主要用于灌溉）等扩张性开发活动影响，莱塔巴河的水资源已遭到严重过度开发。本研究结果表明，大莱塔巴流域内自给农业的灌溉年需水量约为200万立方米，月需水量中位数不超过30万立方米。这意味着小农户农业的灌溉需水量仅约为环境流量预估需求的十分之一。但小规模农户正面临持续扩大的作物水分缺口，这对灌溉农业造成了限制，尤其在旱季情况更为突出。鉴于需为生态目标可持续维持环境流量，作物水分缺口恐将进一步扩大，进而损害灌溉农业的可持续性。若通过上游水坝主动补水，同时保障生态用水与以生计为导向的河道流量，则可完全消除作物水分缺口。但由于存在用水竞争，此类补水方案并无切实保障。