Hydrology Forecasting using SWAT Hydrologic Models for the 2014 California Drought Guobiao Huang and Francis Chung Bay-Delta Office CA Department of Water Resources CWEMF Annual Meeting March 10, 2015
Outline Approach Water Year 2014 review Water Year 2015 outlook (Feather River) Limitation and further work Summary 2
Approach Daily SWAT models were calibrated with observed historical unimpaired streamflow data. Up to date model update is possible by using PRISM daily climatic data. For current water year, climatic data (daily Precip, Tmax, Tmin) for remaining days of the year are sampled from historical data. Assume independence of month to month Precip. 3
Sacramento Sub watersheds Ten separate daily SWAT models (1915-2013) Model calibration with monthly observed unimpaired streamflow at watershed outlets. with SWAT Models 4
Use of PRISM Daily climatic data PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu Feather River, sub basin #31 PRISM 02-07-2015 (mm/day) 5
Steps in Hydrology Forecasting (e.g. WY2015) Update SWAT models With PRISM daily Precip, Tmax and Tmin data Each watershed has tens of sub-basins Spatially averaged over sub-basins SWAT model runs Each watershed has its own combination of current year climatic data Results 100 realizations of current year hydrology Post-processing for frequency and daily cumulative plots 6
Illustration of Forecasting October-January: simulated common historical period February September: 100 possibilities using 1915-2014 weather data. 7
WY2014 Review At early February 2014, the drought was very severe, installation of the delta emergency temporary rock barriers was proposed. Drought modeling was performed to assist decision making. Historical critical dry years: 1976, 1977, 1924, and 1931. 8
Changing Drought condition: Lake Shasta 9
Feather River Lake Oroville 10
Feather River Lake Oroville 11
Feather River Lake Oroville 12
Feather River Lake Oroville 13
Feather River Lake Oroville 14
Feather River Lake Oroville 15
Yuba River 16
American River Folsom Lake 17
Cumulative Streamflow for water years (Feather River basin ) 18
Feather River Basin WY2015 Outlook Another dry year? very likely 19
WY2015 Outlook -Precip max WY2015 WY2014 20
WY2015 Outlook -Streamflow 21
WY2015 Outlook -Streamflow Very small chance to become a normal year. 22
Model Limitation and Potential Bias Model calibration is optimized to average conditions, recalibrate the models to better fit drought years may improve drought forecast performance. Antecedent soil water storage has significant impact on runoff generation, current models tend to overestimate runoff following extended drought periods? 23
Even during extended drought, simulated soil water has annual cycle. 24
Summary Calibrated daily SWAT models can be used to simulate future drought scenarios. Daily PRISM data is very useful in updating model climatic inputs. Impact of soil water initial condition following extended drought period may not be adequately simulated by current generation of models such as SWAT. Automation of data processing and model runs, postprocessing is necessary. 25
Hydrology Forecasting using SWAT Hydrologic Models for the 2014 California Drought Guobiao Huang (DWR) and Francis Chung (DWR) During the prolonged drought in Water Year 2014, we made use of the in-house calibrated physically-based SWAT watershed models for the Sacramento River basin, and performed the forecast of simulated daily unimpaired streamflow for remaining of the WY2014 to assist DWR drought operation and planning. Weekly update of model input of climate data was made using the 4km grid PRISM daily data. Historical climate data from 1915 to 2013 were then used for remaining of the WY2014 to make probabilistic forecasts. The modeled streamflow frequency and probability plots were then used to inform current drought situation/position in comparison to historical critical dry years (1976, 1977, 1924, and 1931). It was good added information for drought decision making such as the installation and timing of the delta emergency temporary barriers. 26