jpolchlo
commented
2 years ago I've been reading some things and investigating the available NWM variables to try and figure out how to approach this problem.
The wording of the problem as being a "closure of the water budget" sent me into sources about what the water budget is. For the most part, this is as said in the equation in the issue description: there is an accounting identity tracking the paths by which water moves through a system. Closure of this budget seems to be related to the act of estimating a term using estimates of the other variables. This is a very broad statement and can operate on the level of getting an external estimate of the terrestrial water storage, and backing out, say, $ET$ from other estimates of $R$ and $P$. Estimates of $S$ could come from the GRACE mission, say.
The focus on the term "water budget" however, led me down a track of estimating the quantity of water available for exploitation. This proved interesting, but not extremely fruitful, since these water budget tasks have much to do with estimating the part of discharge from an area of interest that can be captured, as this is the only way to harvest water in a sustainable fashion. This appears to be a much more complex question, and not solvable using the NWM alone.
After this roundabout tour of the subject of water budgets, though, it occurred to me that the available variables in the NWM exclude estimates of precipitation. We can take a more direct approach of applying the water balance identity to back out an estimate of precipitation for each analytical unit. This is a tractable and clear application of the NWM data.
We can do this operation using the retrospective data (and possibly on short-term data) for any timestep or interval, and we can scope this operation to whichever analytical unit seems reasonable.
I'll need a bit more time to identify the steps that will need to be taken in order to accomplish this goal, but at least there is a goal to strive for now.
References
- Healy, R. W., Winter, T. C., LaBaugh, J. W., & Franke, O. L. (2007). Water budgets: foundations for effective water-resources and environmental management (Vol. 1308, p. 90). Reston, Virginia: US Geological Survey.
- Bredehoeft, J. D., Papadopulos, S. S., & Cooper, H. H. (1982). Groundwater: The water budget myth. Scientific basis of water resource management, 51, 57.
elizawallace
Task 1-4 (#29) is concerned with demonstrating our ability to use National Water Model data to compute a solution to the water budget closure problem. As described in a response to the issue above, the water balance equation is $$\frac{\mathrm{d}S}{\mathrm{d}t} \equiv P - R - ET$$ where $S$ is water storage (predominantly groundwater and lake water), $P$ is precipitation, $R$ is runoff (net outflow), and $ET$ is evapotranspiration. These quantities are assessed on the basis of some areal unit, which is probably a HUC of some sort.
The underlying question here is hard to put a finger on. What, exactly, is the computational task to undertake here? This issue is about finding an answer to this query. We'll be done with this issue when we have an actionable plan to move toward an application of the water balance equation.