Changing Water Resources
How is climate change expected to impact state water resources?
Figure 1: Drip irrigation provides water to sunflower seedlings.
Climate change is already impacting California's water resources. During the past century, mean annual temperature has increased by roughly 0.6 - 1.0oC. This has led to a significant reduction in spring snowpack in the Sierra Nevada and a loss of approximately 1.5 million acre-feet of snow water storage , , . By 2020, demand for water will exceed available supply by more than 2.4 million acre-feet in average rainfall years and up to 6.2 million acre-feet in dry years . Global climate models suggest that the current warming trend will accelerate, with temperatures expected to increase by 2 to 6oC by the end of this century . While there tends to be less agreement among climate models as to whether annual precipitation in California will increase or decrease, inter-annual variability is already on the rise and projected to increase further during the latter half of this century . The magnitude and uncertainty associated with such changes present significant challenges to local water resource managers as they plan for the future.
What is WEAP and how can WEAP help?
The Water Evaluation And Planning (WEAP) system is a modeling system that helps water resource managers integrate climate change projections into their decision making process . WEAP is commonly used to model a watershed's climate, hydrology, land use, infrastructure, and water management priorities. Since WEAP models can be developed at both the regional and local scale, they provide opportunities to improve communication between water managers and climate scientists, and ultimately enhance the community's adaptive capacity . With this goal in mind, a WEAP model of the Cache Creek watershed, which supplies water to the Yolo County Flood Control and Water Conservation District (YCFCWCD), has recently been developed through collaboration between scientists and local water resource managers at the Stockholm Environment Institute, UC Davis and the local irrigation district .
Figure 2: Map of study area modeled using WEAP.
What are the projections for climate change and its impacts on local water resources?
Table 1 shows historical trends in the frequency of years below full allocation and years with no allocation. It also gives projections for the same variables in the near term, midterm, and far term. Relative to the historic period, the model predicts a lower frequency of years below full allocation, in the near and mid-term, when looking at changes related solely to climate (B1 and A2 scenarios). These scenarios did, however, result in more frequent years below full allocation (or shortfalls) in the far-term (Table 1) and suggest that the number of years receiving no allocation will increase gradually with time, particularly during the latter half of the century.
Along with the decreasing supply, scientists predict that due to the warming climate, farmers' demand for water will increase. Relative to the historical period, B1 and A2 scenarios suggest that demand will increase by 102 and 120 thousand acre feet respectively. This is an increase in irrigation demand within the district's service area of approximately 26-32% due to climate alone.
Table 1. Comparison of water years below full allocation, annual irrigation demand, and annual groundwater supply for the historical and future periods under various climate and adaptation scenarios. The B1 and A2 climate scenarios are derived from downscaled projections of the IPCC's GFDL general circulation model. Each time period represents 30 years.
Adaptation 1 is based on land use projections derived from an econometric model. Since the econometric model only covered the 2009-2050 period only midterm data are presented. Adaptation 2 uses hypothetical land use projections which assume a gradual shift towards a more diverse and water efficient cropping pattern. Adaptation 3 combines the diversified cropping pattern and with a projected increase in irrigation technology adoption.
Figure 3. Precipitation, temperature, irrigation demand and groundwater supply for the Yolo County Flood Control and Water Conservation District during the historical, near term, midterm, and far term periods according to the climate only scenario without adaptation strategies. Projections for the future periods are simulated in WEAP using downscaled climate data from the IPCC's B1 and A2 emissions scenarios.
What are some adaptation strategies to keep irrigation demand near historical levels?
Using WEAP, four different scenarios were evaluated for Yolo County; one examining the impact of climate only and the remaining three examining climate combined with a particular adaptation strategy:
- Climate only: potential impacts of climate change alone, under the two IPCC emission scenarios (B1 and A2). Land use is held constant at the 2008 baseline.
- Climate and dynamic cropping (Adaptation 1): corresponds to a dynamic econometric model which simulates future cropping patterns based on B1 and A2 climate sequences.
- Climate and crop diversification (Adaptation 2): corresponds to modeling of a hypothetical diversified cropping pattern, under the two climate scenarios, A2 and B1.
- Climate, crop diversification, and technology (Adaptation 3): corresponds to modeling of hypothetical improvements in irrigation technology and crop diversification under climate scenarios A2 and B1.
Table 1 and Figure 3 compare the difference in irrigation demand among the three adaptation scenarios relative to the historic period and climate only scenarios. Under Adaptation 1, demand varies to a small extent above and below the zero lines for both A2 and B1. This suggests two things: first, that cropping patterns predicted by the econometric model, which are based on historic weather and market drivers, do not have as much impact on irrigation demand as climate alone. Second, the fact that demand in the B1 scenario shows a slight increase with Adaptation 1 implies that the cropping trend projected by the econometric model may be less water efficient than the current copping pattern. In short, the econometric model predicts a cropping pattern that is likely to be the most economical rather than the most water efficient.
Adaptation 2 scenarios also show increased demand compared to the historical baseline across all periods and emissions scenarios (Table 1; Figure 3). However, the model indicates that the increase in demand can be minimized to some extent by shifting to a more diverse and water efficient cropping pattern. That said, the marginal savings towards the end of the century are still less than half of the increase in demand due to climate change alone (Figure 2). Adaptation 3 scenarios also show a near-term demand greater than the historical period. However, as the diversified cropping pattern and improvements in irrigation technology are gradually implemented, far-term demand declines to approximately 12% below the historical mean for both the B1 and A2 climate sequences (Table 1; Figure 3). This illustrates that "game-changing" water savings can occur through a combination of irrigation technology improvement, and efficient, diversified cropping patterns.
Figure 4. Difference in projected irrigation demand for three adaptation scenarios relative to the impact of climate alone 2009-2099. The B1 and A2 climate scenarios are derived from downscaled IPCC projections.