Based on the paper from the word document I have attempted to understand the summary of rewater gap. There were a couple spots that I had to re-read a couple times and so I tried to make them more accessible.
I am currently still trying to make sense of the „Actual net abstraction from groundwater“ part, but in time will understand that as well and will be able to explain it.
Computation of actual net abstractions from surface water and groundwater
In WaterGAP, water usage across five sectors is simulated, differentiating between groundwater and surface water sources. The model assumes how much water is returned to either surface water or groundwater.
Unlike other hydrological models, the impact of human water use on water storage and flow is calculated by subtracting net abstractions from either groundwater (groundwater use) or surface water bodies (surface water use). Priority is assigned to the water body type in ascending priority where global lakes/reservoirs are 1, rivers 2 and local lakes/reservations 3.
Net abstractions are the difference between the water abstractions from a storage compartment (groundwater or surface water) and the water returned to that compartment (Döll et al., 2012). This concept is used to gauge the impact of human water use on water storage. Thus, the impact of irrigation on soil water storage is neglected in WaterGAP.
Net abstractions (NA) can be either positive or negative; negative occurs when the water returned to a compartment surpasses the amount taken out. Potential net abstraction from groundwater (NApot,g) and potential net abstraction from surface water (NApot,s), which do not take into account water availability, are computed in GWSWUSE (Groundwater-Surface Water Use) and are input to WGHM (WaterGAP Global Hydrology Model). The sum of these potential net abstractions reflects the potential consumptive water use – the portion of taken water that's evaporated or transpired during usage.
1 Computation of potential net abstractions from surface water and groundwater
Only three sectors are assumed to use groundwater as their source: irrigation, domestic, and manufacturing. A fraction of the water returned from irrigation (whether from surface or groundwater) refills groundwater at a rate of frgi, while the rest directly flows back to surface water bodies. For the other sectors, their return flows go directly to the surface water. Implementing these assumptions, the module GWSWUSE computes NApot,g as the sum of the three sectors using groundwater minus the artificial groundwater recharge.
NApot,g = [WApot,g,irri + WApot,g,dom+WApot,g,man] - [frgi (WApot,g,irri-CUpot,g,irri+WApot,s,irri-CUpot,s,irri)] (1)
with:
WApot: potential water abstraction, in [km3/month],
CUpot: potential consumptive use, in [km3/month],
NA: net abstraction, in [km3/month],
frgi: fraction of return flow (WA-CU) from irrigation to groundwater,
and the indices:
g: groundwater,
s: surface water,
irri: irrigation,
dom: domestic,
man: manufacturing.
The subtracted artificial groundwater recharge is the returned flow from irrigation with surface water or groundwater to groundwater (Döll et al., 2012).
!Note
For computations in WaterGAP, net abstractions are converted from [km3/month] to [km3/day]
For water uses where the source of the water and the destination for the return flow are surface water bodies, only the consumptive use needs to be included in the computation of NApot,s. This applies to water use for cooling thermal power plants, livestock, as well as surface water use in the domestic and manufacturing sectors.
Thus, NApot,s is computed as:
NApot,s = [CUpot,liv + CUpot,thermal + CUpot,s,dom + CUpot,s,man + WApot,s,irri] – [(1-frgi) (WApot,g,irri-CUpot,g,irri+WApot,s,irri-CUpot,s,irri) + (WApot,g,dom-CUpot,g,dom+WApot,g,man-CUpot,g,man)] (2)
with:
liv: livestock,
thermal: cooling of thermal power plants.
1 Actual net abstraction from surface water
The demand for groundwater abstractions is always fulfilled in WaterGAP, assuming an unlimited groundwater volume, which differs from the demand for surface water abstractions.
If the potential net abstraction from surface water (NApot,s), is positive, fulfilling the demand might not be possible due to water scarcity in the surface water bodies of the grid cell.
There are three options for managing this situation by spatially shifting parts of NApot,s to other grid cells.
With the 'riparian water supply' option, if the demanding cell is situated along a global lake or reservoir, NApot,s can be fulfilled from the storage of the lake or reservoir, if possible.
!Note
In WaterGAP this is achieved by computing storages in the output cell.
Alternatively, with the 'neighboring cell water supply' option, any accumulated unsatisfied potential net abstraction from surface water can be satisfied from a neighboring cell with available supply.
Lastly, in the 'delayed water supply' option, surface water demands that couldn't be met on a given day are shifted to a later time in the year (Müller Schmied et al., 2021, p. 1050).
1.1 Riparian water supply option
If the demand cell is a riparian cell of a global lake or reservoir, NAs is satisfied from the lake/reservoir storage if possible. For this purpose, the NApot,s values of all riparian cells are aggregated for each time step if they are positive and then assigned to the outflow cell, effectively subtracting them from the lake/reservoir storage of the outflow cell.
If satisfaction is impossible, the not-satisfied part from the outflow cell is proportionally redistributed to the riparian cells, right after calculating the global lake/reservoir storage. The proportional contribution of each riparian cell to the aggregated demand in the outflow cell is employed to distribute the unmet demand to the riparian cells.
The actual net abstraction from surface water in the global lake/reservoir outflow cells, resulting from NApot,s in riparian demand cells (net_abstraction_sw_for_riparian_cells), and the part of the potential net abstraction from surface water in the riparian demand cell that is supplied from the global lake/reservoir outflow cell (net_abstraction_sw_from_outflow_cell) can be written out.
1.2 Neighboring cell water supply option
Unsatisfied surface water demand from all other cells can be taken by the neighboring cell possessing the largest river and lake/reservoir storage, simulating the effect of water transfers. However, within each cell 'i', the initial priority is to satisfy the water demand of that specific cell 'i' from its own water storage. The secondary priority is to address water demand allocated from neighboring cells using the water storage within cell 'i'. If not all the unsatisfied demand of the demand cell can be fulfilled in the supply cell, the unsatisfied demand is reassigned back to the original demand cell.
In both scenarios, the Net Abstraction of the demand cell decreases, while the potential net abstraction (NApot,s) and NA of the supply cell increases.
When unsatisfied NA of the demand cell can be satisfied within the supply cell, Net Abstraction for Groundwater (NAg) in the demand cell remains constant, as the complete return flow from irrigation with surface water takes place in the demand cell.
In such instances, the sum of NAg and NAs in each grid cell no longer equals the total actual consumptive water use in both the supply and demand cells.
The actual net abstraction from surface water in the supply cell, due to the potential net abstraction from neighboring demand cells (net_abstraction_sw_for_neighbor_cells), and the part of the potential net abstraction from the surface water demand cell that is supplied from the supply cell (net_abstraction_sw_from_supply_cell) can be written out.
1.3 Delayed water supply option
Temporal distribution, aims to compensate that WaterGAP likely underestimates demand satisfaction, by allowing delayed satisfaction of daily surface water demands. This underestimation of stored water likely arises from the generic reservoir algorithm and an underestimation of the water storage, such as through small tanks and dams (Müller Schmied et al., 2021, p. 1050).
Even after spatially redistributing unsatisfied Net Abstractions (NAs), if some NAs remain unsatisfied, it is possible to satisfy them until the end of the calendar year.
Unsatisfied NAs for a grid cell are registered by adding them to the variable "accumulated unsatisfied potential net abstraction from surface water" (AccUnNApot,s) at the end of each time step. At the beginning of the next time step, AccUnNApot,s is added to NApot,s for that day. It is then attempted to satisfy AccUnNApot,s by subtracting it from the surface water storages, either increasing or decreasing AccUnNApot,s.
The daily unsatisfied net abstraction from surface water (UnNApot,s) for a grid cell is calculated as AccUnNApot,s(t) minus AccUnNApot,s(t-1) at the end of each time step.
If the result is positive, less water than demanded can be taken from surface water on that day.
If the result is zero, NAs equals NApot,s.
If the result is negative, more surface water is abstracted than demanded on that day.
2 Actual net abstraction from groundwater
In the computation of NApot,g in GWSWUSE, it is assumed that the total irrigation demand through surface water can be fulfilled by the water available in surface water bodies, including rivers.
Since net abstractions from groundwater depend on the return flows from irrigation via surface water (with other sectors assumed to return exclusively to surface water bodies, following Döll et al., 2012), potential net abstraction from groundwater, NApot,g, must be adjusted under certain conditions.
This adjustment is required when, for every cell and time step, the actual net abstraction from surface water for irrigation is smaller than NApot,s for that day, or when due to the delayed water use option, more surface water is abstracted for irrigation than demanded for that day.
The former means, that not all surface water demand can be fulfilled even after spatial redistribution. The later occurs whenever the actual net abstraction from surface water on a specific day differs from the potential net abstraction computed in GWSWUSE.
It is assumed in WGHM that the irrigation water abstraction is reduced in priority, and fulfilled only after non-irrigation demands are fulfilled. The actual net abstraction from groundwater NAg is computed in each time step based on NApot,g(t) and UnNApot,s(t-1), using the equations to compute NApot,g and NApot,s as described in Döll et al. (2012).
Three combinations become possible:
UnNApot,s(t-1) is positive and WApot,s,irri(t) >0
UnNApot,s(t-1) is positive and WApot,s,irri(t) = 0
UnNApot,s(t-1) is negative
UnNApot,s(t-1) is positive and WApot,s,irri(t) >0
UnNApot,s(t-1) is positive and WApot,s,irri(t) = 0
Based on the paper from the word document I have attempted to understand the summary of rewater gap. There were a couple spots that I had to re-read a couple times and so I tried to make them more accessible. I am currently still trying to make sense of the „Actual net abstraction from groundwater“ part, but in time will understand that as well and will be able to explain it.
Computation of actual net abstractions from surface water and groundwater In WaterGAP, water usage across five sectors is simulated, differentiating between groundwater and surface water sources. The model assumes how much water is returned to either surface water or groundwater. Unlike other hydrological models, the impact of human water use on water storage and flow is calculated by subtracting net abstractions from either groundwater (groundwater use) or surface water bodies (surface water use). Priority is assigned to the water body type in ascending priority where global lakes/reservoirs are 1, rivers 2 and local lakes/reservations 3.
Net abstractions are the difference between the water abstractions from a storage compartment (groundwater or surface water) and the water returned to that compartment (Döll et al., 2012). This concept is used to gauge the impact of human water use on water storage. Thus, the impact of irrigation on soil water storage is neglected in WaterGAP. Net abstractions (NA) can be either positive or negative; negative occurs when the water returned to a compartment surpasses the amount taken out. Potential net abstraction from groundwater (NApot,g) and potential net abstraction from surface water (NApot,s), which do not take into account water availability, are computed in GWSWUSE (Groundwater-Surface Water Use) and are input to WGHM (WaterGAP Global Hydrology Model). The sum of these potential net abstractions reflects the potential consumptive water use – the portion of taken water that's evaporated or transpired during usage.
1 Computation of potential net abstractions from surface water and groundwater Only three sectors are assumed to use groundwater as their source: irrigation, domestic, and manufacturing. A fraction of the water returned from irrigation (whether from surface or groundwater) refills groundwater at a rate of frgi, while the rest directly flows back to surface water bodies. For the other sectors, their return flows go directly to the surface water. Implementing these assumptions, the module GWSWUSE computes NApot,g as the sum of the three sectors using groundwater minus the artificial groundwater recharge. NApot,g = [WApot,g,irri + WApot,g,dom+WApot,g,man] - [frgi (WApot,g,irri-CUpot,g,irri+WApot,s,irri-CUpot,s,irri)] (1) with: WApot: potential water abstraction, in [km3/month], CUpot: potential consumptive use, in [km3/month], NA: net abstraction, in [km3/month], frgi: fraction of return flow (WA-CU) from irrigation to groundwater, and the indices: g: groundwater, s: surface water, irri: irrigation, dom: domestic, man: manufacturing. The subtracted artificial groundwater recharge is the returned flow from irrigation with surface water or groundwater to groundwater (Döll et al., 2012).
!Note For computations in WaterGAP, net abstractions are converted from [km3/month] to [km3/day]
For water uses where the source of the water and the destination for the return flow are surface water bodies, only the consumptive use needs to be included in the computation of NApot,s. This applies to water use for cooling thermal power plants, livestock, as well as surface water use in the domestic and manufacturing sectors. Thus, NApot,s is computed as: NApot,s = [CUpot,liv + CUpot,thermal + CUpot,s,dom + CUpot,s,man + WApot,s,irri] – [(1-frgi) (WApot,g,irri-CUpot,g,irri+WApot,s,irri-CUpot,s,irri) + (WApot,g,dom-CUpot,g,dom+WApot,g,man-CUpot,g,man)] (2) with: liv: livestock, thermal: cooling of thermal power plants.
1 Actual net abstraction from surface water The demand for groundwater abstractions is always fulfilled in WaterGAP, assuming an unlimited groundwater volume, which differs from the demand for surface water abstractions. If the potential net abstraction from surface water (NApot,s), is positive, fulfilling the demand might not be possible due to water scarcity in the surface water bodies of the grid cell. There are three options for managing this situation by spatially shifting parts of NApot,s to other grid cells.
!Note In WaterGAP this is achieved by computing storages in the output cell.
1.1 Riparian water supply option If the demand cell is a riparian cell of a global lake or reservoir, NAs is satisfied from the lake/reservoir storage if possible. For this purpose, the NApot,s values of all riparian cells are aggregated for each time step if they are positive and then assigned to the outflow cell, effectively subtracting them from the lake/reservoir storage of the outflow cell. If satisfaction is impossible, the not-satisfied part from the outflow cell is proportionally redistributed to the riparian cells, right after calculating the global lake/reservoir storage. The proportional contribution of each riparian cell to the aggregated demand in the outflow cell is employed to distribute the unmet demand to the riparian cells. The actual net abstraction from surface water in the global lake/reservoir outflow cells, resulting from NApot,s in riparian demand cells (net_abstraction_sw_for_riparian_cells), and the part of the potential net abstraction from surface water in the riparian demand cell that is supplied from the global lake/reservoir outflow cell (net_abstraction_sw_from_outflow_cell) can be written out.
1.2 Neighboring cell water supply option Unsatisfied surface water demand from all other cells can be taken by the neighboring cell possessing the largest river and lake/reservoir storage, simulating the effect of water transfers. However, within each cell 'i', the initial priority is to satisfy the water demand of that specific cell 'i' from its own water storage. The secondary priority is to address water demand allocated from neighboring cells using the water storage within cell 'i'. If not all the unsatisfied demand of the demand cell can be fulfilled in the supply cell, the unsatisfied demand is reassigned back to the original demand cell. In both scenarios, the Net Abstraction of the demand cell decreases, while the potential net abstraction (NApot,s) and NA of the supply cell increases. When unsatisfied NA of the demand cell can be satisfied within the supply cell, Net Abstraction for Groundwater (NAg) in the demand cell remains constant, as the complete return flow from irrigation with surface water takes place in the demand cell. In such instances, the sum of NAg and NAs in each grid cell no longer equals the total actual consumptive water use in both the supply and demand cells. The actual net abstraction from surface water in the supply cell, due to the potential net abstraction from neighboring demand cells (net_abstraction_sw_for_neighbor_cells), and the part of the potential net abstraction from the surface water demand cell that is supplied from the supply cell (net_abstraction_sw_from_supply_cell) can be written out.
1.3 Delayed water supply option Temporal distribution, aims to compensate that WaterGAP likely underestimates demand satisfaction, by allowing delayed satisfaction of daily surface water demands. This underestimation of stored water likely arises from the generic reservoir algorithm and an underestimation of the water storage, such as through small tanks and dams (Müller Schmied et al., 2021, p. 1050). Even after spatially redistributing unsatisfied Net Abstractions (NAs), if some NAs remain unsatisfied, it is possible to satisfy them until the end of the calendar year. Unsatisfied NAs for a grid cell are registered by adding them to the variable "accumulated unsatisfied potential net abstraction from surface water" (AccUnNApot,s) at the end of each time step. At the beginning of the next time step, AccUnNApot,s is added to NApot,s for that day. It is then attempted to satisfy AccUnNApot,s by subtracting it from the surface water storages, either increasing or decreasing AccUnNApot,s. The daily unsatisfied net abstraction from surface water (UnNApot,s) for a grid cell is calculated as AccUnNApot,s(t) minus AccUnNApot,s(t-1) at the end of each time step. If the result is positive, less water than demanded can be taken from surface water on that day. If the result is zero, NAs equals NApot,s. If the result is negative, more surface water is abstracted than demanded on that day.
2 Actual net abstraction from groundwater In the computation of NApot,g in GWSWUSE, it is assumed that the total irrigation demand through surface water can be fulfilled by the water available in surface water bodies, including rivers. Since net abstractions from groundwater depend on the return flows from irrigation via surface water (with other sectors assumed to return exclusively to surface water bodies, following Döll et al., 2012), potential net abstraction from groundwater, NApot,g, must be adjusted under certain conditions. This adjustment is required when, for every cell and time step, the actual net abstraction from surface water for irrigation is smaller than NApot,s for that day, or when due to the delayed water use option, more surface water is abstracted for irrigation than demanded for that day. The former means, that not all surface water demand can be fulfilled even after spatial redistribution. The later occurs whenever the actual net abstraction from surface water on a specific day differs from the potential net abstraction computed in GWSWUSE. It is assumed in WGHM that the irrigation water abstraction is reduced in priority, and fulfilled only after non-irrigation demands are fulfilled. The actual net abstraction from groundwater NAg is computed in each time step based on NApot,g(t) and UnNApot,s(t-1), using the equations to compute NApot,g and NApot,s as described in Döll et al. (2012). Three combinations become possible:
UnNApot,s(t-1) is positive and WApot,s,irri(t) >0
UnNApot,s(t-1) is positive and WApot,s,irri(t) = 0
UnNApot,s(t-1) is negative