Drought Resilience JAWRA Article

• Title: Drought Resilience of the California Central Valley Surface Ground Water Conveyance System
• Description: JAWRA Journal Article
• Drought Mitigation - Doc Type: News Article

DROUGHT RESILIENCE OF THE CALIFORNIA CENTRAL VALLEY SURFACE - GROUND WATER - CONVEYANCE SYSTEM
essential to keep land use unchanged in this phase of
analysis in order to understand only the response to
reduced flows under current conditions.
The CDWR is addressing global climate change in
the California Water Plan, Bulletin 160 (CDWR,
2005a). Specified drought scenarios act as an ana-
logue to projected reductions in snowpack- derived
surface water flows. Rather than focus on causes of
global climate change, which are being addressed by
other agencies and research institutions, the CDWR
Water Plan looks at potential impacts of climate
change on water resources in California and strate-
gies for adapting to these changes.
Model Descriptions
Two computer applications developed by CDWR,
the surface water allocation model California
Simulation Model ( CALSIM II) and the integrated
hydrologic model California Central Valley Ground-
water- Surface Water Simulation Model (C2VSIM),
were used for this study.
CALSIM IL The CALSIM model (Draper et al.,
2004) is a general - purpose, network flow, reservoir
and river basin water resources allocation model
developed jointly by CDWR and the U.S. Bureau of
Reclamation. It is used for evaluating operational
alternatives of large, complex river basins. CALSIM
integrates a simulation language for flexible opera-
tional criteria specification, a mixed integer linear
programming solver for efficient water allocation
decisions, and graphics capabilities for ease of use. A
linear objective function describes the priority in
which water is routed through the system and the
constraints set the physical and operational limita-
tions toward meeting the objective. CALSIM maxi-
mizes the objective function in each time period to
obtain an optimal solution that satisfies all con-
straints.
CALSIM was originally designed, and has been
successfully implemented as a planning model of the
State Water Project (SWP) and Central Valley Project
(CVP) system to examine the range of options to
improve supply reliability. The second - generation ver-
sion used here ( CALSIM II) calculates the reservoir
operations and time dependent rim -flow into the Cen-
tral Valley on monthly time steps, providing the
needed boundary conditions to C2VSIM.
C2VSIM. The C2VSIM model (Brush et al., 2008)
was developed as an application of the CDWR's Inte-
grated Water Flow Model (IWFM; CDWR, 2005b,c,
2006). IWFM simulates land - surface processes, surface
water flow and ground -water flow. The land- surface
module computes infiltration and runoff from net pre-
cipitation; consumptive use by native vegetation, irri-
gated crops and urban areas; surface water diversion
and application; ground -water pumping and applica-
tion; infiltration and return flow from irrigation; and
recharge. Surface water flow is simulated as a function
of flow from upstream reaches, tributaries and lakes;
surface runoff; agricultural and urban return flows;
diversions and bypasses; and exchanges with the
ground -water flow system. Horizontal and vertical
ground -water flow are simulated using the Galerkin
finite - element method and a quasi- three - dimensional
approach utilizing the depth - integrated ground -water
flow equation for horizontal flows in each aquifer layer
and leakage terms for vertical flow between aquifer
layers. To the extent that is practical, IWFM directly
incorporates readily available historical and spatial
datasets, including precipitation, the Natural Resource
Conservation Service (NRCS) runoff curve number,
surface water inflows and diversions, land use and crop
acreages.
The C2VSIM model simulates land- surface pro-
cesses, ground -water flow and surface water flow in
the alluvial portion of the Central Valley (Figure 1)
using a monthly time step. C2VSIM covers an area of
51,394 km (19,834 mi and incorporates 1,392 nodes
forming 1,393 elements and 3 layers, 431 stream
nodes delineating 74 stream reaches with 97 surface
water diversion points, 2 lakes, and 8 bypass canals
(Figure 1A). Surface water inflows are specified for
35 gaged streams and simulated for ungaged small
watersheds. The model area is divided into 21 subre-
gions (Figure 1B), and ground water and surface
water are allocated to meet monthly water demands
in the land- surface process within each subregion.
Regional -scale parameter values were calibrated
using the PEST parameter estimation program (Doh-
erty, 2005) for the 25 -year period 1975 -1999, using
ground -water head observations at 221 wells, paired
ground -water head observations for calculating verti-
cal head gradients at nine locations, monthly river
flow observations at seven locations, and stream -aqui-
fer interaction values at 65 locations along 33 river
reaches. The preliminary calibration produced
hydraulic parameter values that reflect the geologic
composition of subregions within the Central Valley.
The average difference between simulated and
observed ground -water heads for water years 1975-
1999 was 4.43 m (13.5 ft), the RMSE was 24.1 m
(73.4 ft), and the RMSE /range was 11 %. The ground-
water heads produced by the model are considered
reasonably accurate given the discretization of the
finite element grid, in which the average spacing
between model nodes is on the order of 8 km
(5 miles), and the areal extent of the water budget
subregions. Simulated and observed stream to
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 859 JAWRA