Water balance and subarea connections
Nandini Shahi
Hi,
I am using APEX v.1501 and APEXEditor to make a model of the water quality of a catchment, with focus on the N, P, C and water exchanges. I am trying to get the water balance for each subarea using output from .SAD file. The current result is using an example dataset shared previously by the APEX Modeling Team. I am also modifying this input file to set it up for the catchment I am working on but have been testing the water balance on the original data. I started with the equation previously posted on this forum. Since there is no reservoir, the equation reduces to:
(initial – final) soil water content (SW) + (initial-final) ground water storage content (GWST) + precipitation (PRCP) + water yield into the subarea (WYLD) - water yield that leave the subarea (WYLD from headwater subarea, if) - evapotranspiration (ET) - deep percolation (DPRK) + irrigation (IRGA) - irrigation distribution loss (IRDL) - final soil water content in litter(SWLT)
I could not find an output variable from subarea for evaporation from floodwater and inflow to rootzone from water table so haven't been able to include them in the calculation. I am unable to get the water balance close to zero. Instead, I get a rather high residual using the above equation (~400mm for precipitation of ~3000mm). The error becomes higher when I include the water yield input from another subarea for downstream subareas. I checked for ISTA=0, and could not find other parameters that might be affecting the balance.
A graph of residuals for a headwater subarea looks like this:
Does anyone have any suggestions on what the problem could be?
Secondly, I am facing some difficulty in understanding how the model interprets connections between subareas. For instance, in this input file, 1 and 2 are headwaters, 3 is downstream with a negative watershed area (WSA), so 2 would join the outflow from 3 but where does water from 1 go? Does water directly go into the outflow if a downstream subarea or a negative WSA headwater subarea does not follow headwater subarea data?
The associated input and output files are available here: https://drive.google.com/file/d/1XgDofQLR4KPvGolTGr49P_WAQlv7D176/view?usp=sharing
Thanks in advance, Nandini
Nandini Shahi
Hi,
I tried to find why the error is occurring and I noticed the following issues. If anyone has faced similar problems, can you kindly help me resolve this?
1. The error in the water balance follows the pattern of the sum of the groundwater storage (GWST) and soil water storage (SW) (Fig.1). Using the water balance equation in the previous post, I calculated the error with change in storages, but the error follows the pattern of the total storage values and not the difference in storage values.
2. The SW storage stores large amounts of water (>300 mm), not allowing the groundwater to recharge. The daily changes in the soil reservoir also show sudden, unreasonable jumps. This affects the GWST where the water content becomes 0 for a long period leading to zero runoff and zero water yield (Fig.2 and 3).
I saw a previous post where the RZSW is explained to be negative because it is the root zone soil moisture available for plant uptake. But RZSW values are exactly the same as SW values from the .SAD file. So how can I get the actual soil water content values? Is there a problem in the SW variable itself or is it the functioning of SW? Are there variables to control the maximum storage in the SW?
3. Since I am trying to model a catchment in the Netherlands, where the water table is at shallow depths of 0.5 -1 m, upward groundwater seepage has a significant role in the nutrient concentrations as well as in the soil processes. Based on my understanding from the manual, water table parameters are not used for soil water processes except for crop growth. In that case, how can I simulate the upward seepage process from the groundwater? Is there a way to artificially simulate the upward dynamics?
Thank you.
Nandini
