Farmers like to know how their crops are performing and how much water they require. Especially when little water is available and farmers must apply their water as efficiently as possible. With the Triangle method and using a blended satellite data and drone imagery approach, FutureWater developed a field-scale dryness index and crop evaporation product. This product is based on a relatively simple method and requires only few data inputs. The product can be produced quickly in data scarce regions and can be used to inform farmers about the relative moisture status of their field and water use requirements of their crops.

To create a clear picture of evaporation at field scale, hydrological models could be used. However, this requires a lot of input data which is often a huge challenge in Sub-Saharan African countries, where data is scarce. Satellite and drone remote sensing data can help mitigate these issues. Applying the Triangle method to MODIS satellite imagery, we calculated the Temperature Vegetation Dryness Index (TVDI) for a northern part of Ghana. The TVDI indicates, with values between 0 and 1, how dry the soil is based on the relationship between Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST). Essentially, if the TVDI is close to 1, the soil is dry and if the TVDI is close to 0, the soil is wet.

With an affordable, commercial drone, equipped with a near-infrared and thermal camera, we obtained high-resolution NDVI and LST maps of a cropped field and blended these data with the MODIS-derived TVDI. The satellite data was used to define the shape (i.e., dry and wet edge) of the Triangle, but the TVDI was calculated using the measurements of the drone. Despite some challenges, we concluded this is a reliable and relatively simple method to get an indication of the water requirement of crops at field scale. The application is relatively expensive to implement as a geo-service to an individual (smallholder) farmer, but the method can easily be upscaled to cover much large areas, so it becomes available to many farmers simultaneously. This would greatly reduce the cost of the service and allow for better coordination of water resources, which can benefit the entire community.