Budgets
PD Capel
Hi. I would like to construct annual water and nitrogen budgets from WinAPEX model output. I have a few questions on how to use the APEX variables to accomplish this. Thank you, Paul
For these questions, please consider a simple watershed that has only one subarea. There is a single channel in the subarea. The end of this channel is the outlet for the subarea (and the watershed). There are no reservoir or groundwater components. The water and nitrogen that moves through the soil column is lost to the model. It does not contribute to the water or nitrogen in the channel. There is no contribution of water in the channel from groundwater (baseflow).
The important APEX model variables for water and nitrogen budgets might include:
-- Variables for water and nitrogen movement to the channel
Surface runoff (Q and QN, for water and nitrogen, respectively)
Lateral subsurface flow (SSF and SSFN).
Return subsurface flow (RSSF, ?????)
Quick return flow (QRF, QRFN)
Point source flow (PSOQ, PSON)
Flow from drainage system (QDR, QDRN)
-- Variables for water and nitrogen movement to and from the atmosphere
Water from precipitation (PRCP)
Nitrogen in precipitation (RNFX)
Nitrogen from precipitation: The mass of nitrogen (kg/ha) from rain would be = PRCP * RNFX (and adjusted for units).
Evapotranspiration (ET, water only)
Nitrogen volatilization (AVOL, nitrogen only)
-- Variables for water and nitrogen from irrigation
Water from irrigation application (QXM or IRGA)
Nitrogen in irrigation water (CQNX)
Nitrogen from irrigation water: The mass of nitrogen (kg/ha) from irrigation water would be = QXM * CQNX (and adjusted for units).
-- Variables for water and nitrogen movement through and out of the soil column and harvest
Percolation below the root zone (PRK, PRKN)
Percolation (QT, QNO3)
Deep percolation (DPRK, DPRN)
N harvested with crop (YLN)
QUESTIONS:
1) The water in the channel would be comprised of the sum of Q + QRF + PSOQ + QDR + RSSF, correct?
2) Is the sum in question (1) the same as the water yield (WYLD)? For the case where there is only 1 subarea in the watershed, is the sum from question (1) also the same as watershed outflow (QTW)?
3) The overall water budget equation (inputs = outputs + change in storage) for the subarea would be
PRCP + QXM (or IRGA) = ET + channel volume + change in soil water (SW), correct?
4) How is IRGA different from QXM?
5) Is there a variable for annual change in soil water (change in SW, correct?) that can be reported in the annual output file? If not is variables for the change in soil water, is there a variable for the soil water content at the beginning of the time step (annual), so that the change in soil moisture can be calculated outside of APEX?
6) Is the variable for the “change in total N pool” (DW0N), the parallel for nitrogen to my question for water in question (5)?
7) What is lateral subsurface flow (SSF)? Is this the water that moves from a subarea via the subsurface to streams (channels) outside of the subarea? If the answer is yes, then this will not be a contributor to the water in the channel within a subarea, correct?
8) Is return subsurface flow (RSSF) water that come from the subarea and moves via the subsurface to channels within the same subarea?
9) What is the variable for the nitrogen in RSSF water?
10) What are the differences among these three pairs for water and nitrogen:
PRK/PRKN compared to QT/QNO3 compared to DPRK/DPRN?
11) Following the format of the budget equation from question (3), the annual mass budget for nitrogen for the single subarea would be:
Nitrogen inputs (rain, irrigation, fertilizer/manure) = Nitrogen outputs (volatilization, surface runoff, return subsurface, quick return, point source, drainage system, percolation, and harvest) + Change in nitrogen content of soil
Using the APEX variable names:
[N from rainfall (PRCP * RNFX) + N from irrigation (QXM * CQNX) + FN =
[AVOL + QN + (RSSF * N in RSSF water) + QRFN + PSON + QDRN + PRKN + YLN] + [DW0N]
12) Does the variable FN include both chemical fertilizer and manure?
Luca
The answers follow the same numbers of your questions.
1) yes, you are correct.
2) yes, it is the same of water yield. Yes it is the same for the watershed outflow if you have only one subarea.
3) here is how the model calculate the final subarea water balance:
initial soil water content + initial snow + initial reservoir water content + initial ground water storage content + precipitation + water yield into the subarea - water yield that leave the subarea - evapotranspiration - evaporation from flood water - evaporation from reservoir - deep percolation + irrigation - irrigation distribution loss + inflow to root zone from water table + reservoir leakage (goes into ground water) - final soil water content - final snow - final reservoir water content - final ground water storage content - final soil water content in litter
4) QXM and IRGA should be the same. We don't have QXM in the APEX code, did you get it from the theoretical documentation?
5) In the .OUT file you can find the soil characteristics at the beginning and at the end of the simulation. The soil physical and chemical data are reported at the beginning of the .OUT file (search for "soil physical data"), and the column CURNT has the information about the soil water content of each soil layer. Then, you can search for "final soil physical data" to get the information on the soil water content at the end of the simulation.
6) Yes, it is.
7) SSF is the water that move horizontally in the subsoil and it remains in the soil and reach the next (downhill) subarea. A fraction of the water in the soil that moves horizontally became return flow if it reaches the channel.
8) Yes.
9) RSNF.
10) PRK = water that percolates below the root zones and goes to the ground water storage. PRKN = soluble nitrogen in PRK. QT(?) I think you mean Q that is the runoff. QNO3 = soluble nitrogen transported by runoff. DPRK = deep percolation. It is the water that percolate from the ground water storage. This fraction of water leaves the system. DPRN = soluble nitrogen in DPRK.
11) As for the water balance, this is how the model calculates the final nitrogen balance:
initial total N + N added with rainfall - N transported with sediments - N loss in wind erosion - N in runoff - N in SSF that leaves the subarea - N in percolate - N loss by denitrification - N harvested with crop - N loss by volatilization + N added with miner fertilizer (nitrate form) + N added with mineral fertilizer (ammonia form) - final total nitrogen + N fixed by legumes + N added with organic fertilizers + N in SSF into the subarea - N in drainage water outflow - N burn loss - soluble N in quick return flow - N in scraping manure + deposition of mineral N at watershed outlet + point source soluble N load
12) Yes, FN (at least in the .OUT file) includes both chemical and organic fertilizers.
Let me know if you need more information.
Luca