This study will contribute to a central question in hydrology: Where and to which extend evaporated water of a region returns as precipitation in another region?
We developed a process based approach that has been implemented into the Mesoscale model MM5. It allows to tag the moisture evaporating from a certain region into the atmosphere, and to track it until returning to the land surface as precipitation.
The MM5 model and its extensions for ET tagging
- New model variables representing the mixing ratios of the tagged moisture components are defined
- New model equations for the tagged moisture are formulated. They account for atmospheric processes like transport, diffusion, and grid scale phase transitions and precipitation physics
- Original moisture variables and process equations remain. They represent now the sum of tagged and untagged moisture variables
- ”No tagged moisture outside the model domain” is applied as atmospheric boundary condition
In a case study the model is applied to the Lake Volta region (Ghana, West Africa). We selected a two months period from the rainy season (July, 1 to August, 31, 1998). The simulations are performed with a grid resolution of 9 km and are driven by global NCEP-Reanalysis data.
Model domain with terrain height (m) and the tagging area marked by a red rectangle
Most of the simulated precipitation of tagged water appears up to several hundred kilometers northeastwards of Lake Volta. The highest values are found next to the Lake. In the simulated month August 1998 this accounts for more than 6 % of the total precipitation.
Ratio (%) of tagged and total precipitation in august 1998
Current and future work
- Application for evaluation of precipitation recycling ratios
- Extension for further regions, like Poyang Lake in China, and large scale irrigation areas, e.g. in India
- Technical implementation in the WRF modeling system