The choice of scale for balancing water demand and supply in GCAM:
GCAM reconciles water demand and supply at the river basin scale. Water demands are initially
computed at either the geopolitical region or agro-ecological zones and disaggregated to the
basin scale. A mapping structure is implemented to dynamically allocate water demands from
their native spatial units to the river basin scale. The mapping structure establishes the allocation
of water demands based on the results of a spatial downscaling algorithm (Hejazi et al. 2014b)
that downscales water demands to the grid scale (0.5x0.5 degree spatial resolution) and upscale
to the basin scale. This procedure was applied to the base year (2005) and was kept constant for
future time periods. Water availability is calculated at the grid scale and then aggregated to the
basin scale through the spatial movement of water (i.e., river routing).
The choice of the basin size and scale at which water demands and supplies are resolved will
have implications for understanding the impact of water use. Allocation of water among the
various users at the grid scale is unattainable in this economic framework as it is not possible to
fully represent integrated economic decision making for all goods and services at the grid scale.
On the other hand, we recognize that the aggregation of water demands to large regions may
underestimate the possibility of cascading impact of binding water constraints at the local level.
The use of sub-basins brings about additional challenges of upstream and downstream
interdependencies, and mismatch in spatial and temporal scales within the GCAM regions and
AEZs.
Supplies and demands of are aggregated to the annual temporal scale and GCAM supplies and
demands for all goods and services are resolved at 5-year annual intervals. Seasonal and monthly
variations in water supply and demand are not currently represented. The impact of these
variations, however, depends not only on the flexibility for demand-side management of water
and the reservoir storage capacity that mitigate their potential impact, but also on the flexibility
of the overall economic system in responding to the volatility in the production of agricultural
and industrial commodities, energy services, and municipal demands.
As an initial approach to balancing water demand and supply, we have chosen the river basin and
annual time-step as the appropriate common scale that allows for estimates of water supply and
demand where the economic decision integrates information on all goods and services, including
water availability. Our initial effort is focused on understanding the major long-term regional and
global drivers that lead to gross imbalances in the supply and demand for water resources.
Identifying the most appropriate spatial (grid, sub-basin, or basin) and temporal (daily, monthly,
or yearly) scales for reconciling water demands and supplies in a global integrated assessment
model is an ongoing focus of our continuing research.
Table S1: Energy cost of pumping groundwater ($/m3)
No. GCAM Region
Percentile
1
5
10
50
90
95
99
1
USA
$0.0001
$0.0011
$0.0025
$0.0225
$0.0806
$0.0958
$0.1215
2
Canada
$0.0000
$0.0003
$0.0009
$0.0082
$0.0317
$0.0482
$0.0822
3
Western Europe
$0.0004
$0.0035
$0.0065
$0.0258
$0.0614
$0.0719
$0.0913
4
Japan
$0.0008
$0.0028
$0.0069
$0.0336
$0.0617
$0.0689
$0.0813
5
Australia & NZ
$0.0006
$0.0017
$0.0031
$0.0207
$0.0565
$0.0684
$0.0948
6
Former Soviet Union
$0.0001
$0.0007
$0.0015
$0.0091
$0.0339
$0.0454
$0.0724
7
China
$0.0002
$0.0009
$0.0026
$0.0224
$0.0693
$0.0827
$0.1053
8
Middle East
$0.0006
$0.0023
$0.0052
$0.0335
$0.0758
$0.0838
$0.0941
9
Africa
$0.0004
$0.0016
$0.0040
$0.0246
$0.0793
$0.1031
$0.1475
10
Latin America
$0.0003
$0.0007
$0.0011
$0.0172
$0.0733
$0.0897
$0.1176
11
Southeast Asia
$0.0002
$0.0007
$0.0013
$0.0262
$0.0744
$0.0844
$0.1041
12
Eastern Europe
$0.0006
$0.0023
$0.0050
$0.0266
$0.0571
$0.0639
$0.0731
13
Korea
$0.0026
$0.0144
$0.0202
$0.0477
$0.0607
$0.0642
$0.0681
14
India
$0.0004
$0.0008
$0.0011
$0.0150
$0.0585
$0.0807
$0.1025
Table S2: Estimated cost of desalination in the US and globe
Desalination
Wangnick/GWI (2005)
Zhou and Tol (2005)
water source
Share (%)
Cost ($/m3)
Global
USA
Global
USA
Sea
56%
7%
1
1
Brackish
24%
51%
0.6
0.6
River
9%
26%
0.6
0.6
Waste water
6%
9%
0.6
0.6
Pure
5%
7%
0.6
0.6
Brine
0%
0%
SUM
100%
100%
0.824
0.628
Table S3. Comparison of representative water price differential between the agricultural and energy sectors
Water Price
Agriculture Water Coefficient (m^3/kg)
($/m^3)
Water Cost ($/kg) Crop Price ($/kg) Water Cost Share
Corn
0.42
0.10
0.042
0.10
43%
Rice
0.80
0.10
0.080
0.66
12%
Wheat
0.58
0.10
Water Price
0.058
0.13
Elect Price
45%
($/GJ)
Water Cost Share
Electricity Water Coefficient (m^3/GJ)
($/m^3) Water Cost ($/GJ)
Coal Conv.
2.24
0.10
0.22
15.26
1.5%
Gas CC
0.20
0.10
0.02
13.71
0.15%
Nuclear
1.65
0.10
0.17
15.89
1.0%
Figure S1. Schematic of GCAM structure with the inclusion of the water system
Figure S2. Schematic of the overall market-based approach across all goods and services in GCAM
(a)
(b)
(c)
Figure S3. (a) Gridded estimates of mean water table depth in meters with global coverage based on the work by Fan
et al. (2013); (b) cumulative distribution of mean water table depth for each of the 14 GCAM regions; (c) the
equivalent cumulative distribution of energy cost associated with pumping ground water for each GCAM region.
Figure S4. Procedure for estimating the total amount of desalinated water at the basin scale in 2005
Figure S5. Schematic of the water allocation mechanism at the basin scale in GCAM
Figure S6. Percent changes in crop productions between the water constrained and unconstrained scenarios by
GCAM region; negative values indicate decrease in crop productions when water is constrained in GCAM.
Figure S7. Percent change in global crop productions (a) and prices (b) for the three major cereal crops (corn, rice,
and wheat) between the water constrained and unconstrained scenarios.