Producing more food for a growing population requires sustainable crop intensification and diversification, particularly in high-potential areas such as the seasonal floodplain wetlands of sub-Saharan Africa (SSA). With emerging water shortages and concerns for conserving these multi-functional wetlands, a further expansion of the cropland area must be avoided as it would entail increased use of blue water for irrigation and infringe on valuable protected areas. We advocate an efficient use of the prevailing green water on the existing cropland areas, where small-scale farmers grow a single crop of rainfed lowland rice during the wet season. However, soil moisture at the onset of the rains (pre-rice niche) and residual soil moisture after rice harvest (post-rice niche) may suffice to cultivate short-cycled crops. We developed a methodological approach to analyze the potential for green water cultivation in the pre- and post-rice niches in the Kilombero Valley Floodplain in Tanzania, as a representative case for seasonal floodplain wetlands in SSA. The three-step approach used open-access remote sensing datasets to: (i) extract cropland areas; (ii) analyze soil moisture conditions using evaporative stress indices to identify the pre- and post-rice niches; and (iii) quantify the green water availability in the identified niches through actual evapotranspiration (AET).

We identified distinct patterns of green water being available both before and after the rice-growing period. Based on the analyses of evaporative stress indices, the pre-rice niche tends to be longer (~70 days with average AET of 20–40 mm/10-day) but also more variable (inter-annual variability >30%) than the post-rice niche (~65 days with average AET of 10–30 mm/10-day, inter-annual variability <15%). These findings show the large potential for cultivating short-cycled crops beyond the rice-growing period, such as green manure, vegetables, maize, and forage legumes, by shifting a portion of the nonproductive AET flows (i.e., soil evaporation) to productive flows in form of crop transpiration. A cropland area of 1452 to 1637 km² (53–60% of the total cropland area identified of 2730 km²) could be cultivated using available green water in the dry season, which shows the significance of such change for food security, livelihoods, and resilience of the agricultural community in Kilombero. A wider application of the developed approach in this study can help identifying opportunities and guiding interventions and investments towards establishing sustainable intensification and diversification practices in floodplain wetlands in SSA.

为不断增长的人口生产更多粮食需要可持续的作物集约化和多样化，特别是在高潜力地区，例如撒哈拉以南非洲 (SSA) 的季节性洪泛区湿地。随着水资源短缺问题的出现以及对保护这些多功能湿地的担忧，必须避免进一步扩大农田面积，因为这将需要更多地使用蓝色水进行灌溉并侵犯宝贵的保护区。我们提倡在现有农田地区有效利用盛行的绿水，小农在雨季种植单一作物的雨养低地水稻。然而，降雨开始时的土壤水分（水稻前生态位）和水稻收获后的剩余土壤水分（水稻后生态位）可能足以种植短周期作物。我们开发了一种方法来分析坦桑尼亚 Kilombero 河谷洪泛区水稻前和水稻后生态位中绿水种植的潜力，作为 SSA 季节性洪泛区湿地的代表性案例。三步法使用开放获取的遥感数据集来： (i) 提取农田区域；(ii) 使用蒸发应力指数分析土壤水分条件，以确定水稻前和水稻后生态位；(iii) 通过实际蒸散量 (AET) 量化已确定生态位中的绿水可用性。(i) 提取农田面积；(ii) 使用蒸发应力指数分析土壤水分条件，以确定水稻前和水稻后生态位；(iii) 通过实际蒸散量 (AET) 量化已确定生态位中的绿水可用性。(i) 提取农田面积；(ii) 使用蒸发应力指数分析土壤水分条件，以确定水稻前和水稻后生态位；(iii) 通过实际蒸散量 (AET) 量化已确定生态位中的绿水可用性。

我们确定了水稻生长期前后不同的绿水模式。根据蒸发胁迫指数的分析，水稻前生态位往往更长（约 70 天，平均 AET 为 20-40 毫米/10 天），但变化也更大（年际变化 > 30%）。水稻后生态位（约 65 天，平均 AET 为 10-30 毫米/10 天，年际变化 <15%）。这些发现表明，通过将一部分非生产性 AET 流量（即土壤蒸发）转移到生产性流量，在水稻生长期之后种植短周期作物（如绿肥、蔬菜、玉米和豆科牧草）具有巨大潜力以作物蒸腾的形式。1452 至 1637 km ^{2的农田面积（已确定的 2730 km 2}总农田面积的 53-60%) 可以在旱季使用可用的绿水进行种植，这表明这种变化对 Kilombero 农业社区的粮食安全、生计和复原力具有重要意义。在本研究中更广泛地应用已开发的方法可以帮助确定机会并指导干预措施和投资，以在 SSA 洪泛区湿地建立可持续集约化和多样化实践。