Rainwater harvesting in catchments for agro-forestry uses: A study focused on the balance between sustainability values and storage capacity

Highlights: • Improved rainwater harvesting suitability model is presented. • The harvested rainwater is to be used in irrigation or wildfire combat. • The model differs from others because it uses dam wall height as evaluation parameter. • The use of small height dam walls can greatly limit irrigable area. • More engineered dams are more suited for larger-scale agro-forestry uses. Rainwater harvesting (RWH) is used to support small-scale agriculture and handle seasonal water availability, especially in regions where populations are scattered or the costs to develop surface or groundwater resources are high. However, questions may arise as whether this technique can support larger-scale irrigation projects and in complement help the struggle against wildfires in agro-forested watersheds. The issue is relevant because harvested rainwater in catchments is usually accumulated in small-capacity reservoirs created by small-height dams. In this study, a RWH site allocation method was improved from a previous model, by introducing the dam wall height as evaluation parameter. The studied watershed (Sabor River basin) is mostly located in the Northeast of Portugal. This is a rural watershed where agriculture and forestry uses are dominant and where ecologically relevant regions (e.g., Montezinho natural park) need to be protected from wildfires. The study aimed at ranking 384 rainfall collection sub-catchments as regards installation of RWH sites for crop irrigation and forest fire combat. The height parameter was set to 3 m because this value is a reference to detention basins that hold sustainability values (e.g., landscape integration, environmental protection), but the irrigation capacity under these settings was smaller than 10 ha in 50% of cases, while continuous arable lands in the Sabor basin cover on average 222 ha. Besides, the number of sub-catchments capable to irrigate the average arable land was solely 7. When the dam wall height increased to 6 and 12 m, the irrigation capacity increased to 46 and 124 sub-catchments, respectively, meaning that more engineered dams may not always ensure all sustainability values but warrant much better storage. The limiting parameter was the dam wall height because 217 sub-catchments were found to drain enough water for irrigation and capable to store it if proper dam wall heights were used.