The global hydrological cycle is a closed system, meaning that the amount of water is fixed. No input, no output. It is just the circulation! Every river basin takes the amount of water it needs for its ecosystem maintenance and return it back to the global hydrological system. For a river system, the input to the basin is precipitation whereas the outputs are the amount of water that the basin return it back to the system. These are the discharge ( river flow), and evapotranspiration driven by the energy balance. As the system changed from the closed system to the open system as moving to the global cycle to basin water cycle, the basin scale at which the hydrological cycle is looked at matters. This mean that the proportion of the components such as the precipitation, discharge, evapotranspiration, storage varies across scale.
The first procedure in modelling the hydrological system is the geometry at which the cycle is estimated. This geometry is extracted from the digital elevation models. These days, they can be easily available from different sources. Based on the objective and purpose of the modelling, the spatial scale of the basin can be ranged from few kilometres to hundreds of kilometres (or continental scale). The digital watershed modelling (DWM) is the pre requisite for modelling, for instance, we are interested at different scales, and we have been working in the following:
- Posina Bain, small scale basin
- Adige scale, large scale basin
- Upper Blue Nile basin, Macro scale
2. Adige river basin
I will not talk about Posina basin in this post. I will have other post about small basin DWM, and hydrological modelling space-time variability. Adige is one of the largest, the second largest basin in Italy(?). It provides water resources to all the Bolzano, Trentino and Veneto region. We have interest to model water resource at this basin, and the first step is the DWM. It is possible to start from the whole basin, and look it into the detail. To work on the maximum detail topographic information, following series of steps as described in other post, DEM need to be partitioned into many detail, for instance, here the Adige is divided into about 1200 HRUs.
|Adige basin partion into 1200 HRUs using JGrass Spatial toolbox|
However, such large numbers of HRUs could computationally be demanding and difficult for data management, particularly if we are interested to the hydrological outputs at each HRUs. For this reason, the basin can be separated into major basin, and the simulation can be take care of at each particular basin, and use some routing system to estimate at the furthest outlet of the whole basin. For instance, Adige basin can be divided into several basin (notice the black divides inside the basin, in to five major basin), and then hydrological simulation can be carried out at each basin, and some sort of routing mechanism can be applied to route to the outlet.
Some of these can be:
The position of Adige-Passirio basin (right) and the topography partitioning into HRU
Rienza basin and its topographic partitioning
Avisio basin and its partition
3. Upper Blue Nile basin
What we have to do if we are interested even larger (very larger ) basin than the Adige ??? For instance Upper Blue Nile basin, the Ethiopia part of the Blue Nile? Let's start from what people already did:
Lake Tana basin where most hydrological studies in the UBN basin is conducted
- e.g Alemseged T. Haile, Tom Rientjes, Ambro Gieske, and Mekonnen Gebremichael, 2009: Rainfall Variability over Mountainous and Adjacent Lake Areas: The Case of Lake Tana Basin at the Source of the Blue Nile River. J. Appl. Meteor. Climatol., 48, 1696–1717. doi: http://dx.doi.org/10.1175/2009JAMC2092.