Alan F. Hamlet Dennis P. Lettenmaier JISAO Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington October, 2004 Weekly and Daily Climate Change Streamflow Scenarios and Estimates of Changing Crop Water Demand

Problem: 1) Monthly naturalized streamflow observations are frequently available for a large number of sites over long periods of time, but availability of weekly and daily observations is typically very limited. 2) Monthly climate change scenarios are useful, but for many studies weekly or daily flows are required (e.g. flood control). Methods are needed to: Produce weekly and daily “observed” records that are consistent from monthly naturalized data. Produce climate change scenarios at weekly and daily time step that are consistent with observed data sets.

Observed Monthly Average Streamflow Data Simulated Daily Streamflow Data Simulated Weekly Streamflow Data Reconstructed “Observed” Daily Record Reconstructed “Observed” Daily Record VIC Hydrologic Model Driven by Observed Temperature and Precipitation Data Monthly average value comes from observed data. The daily and weekly time history come from the simulations. Short Time Step Streamflow Reconstruction Process

Reconstructed Naturalized Weekly and Daily Flows at Palisades Dam for Streamflow (cfs) Weekly Flow Daily Flow

Bias Corrected Monthly or Weekly Climate Change Streamflow Simulated Daily Streamflow Data Daily Time Step Streamflow Scenario VIC Hydrologic Model Driven by Climate Change Temperature and Precipitation Scenario Monthly or weekly data comes from bias corrected simulation. The daily time history used to construct the daily data comes from the simulations. Short Time Step Climate Change Streamflow Scenario

Weekly Climate Change Scenario for Palisades (MPI 2040 “warm and dry”)

Long-Term Estimates of Potential Evapotranspiration from a Reference Crop

Problem: Quantitative, spatially-explicit estimates of evaporation from irrigated crops are needed for: Estimates of future surface water diversions and return flows as a function of climate, irrigation technology, crop type, etc. Estimating losses from aquifers due to groundwater pumping for irrigation and aquifer recharge due to surface water application.

Methods: A well-tested and frequently used method is to estimate the “potential evaporation” (PotET) from a well-watered reference crop (e.g. mature alfalfa), and then relate this to the PotET for other crops using linear factors that vary with crop type and season: Actual Crop PotET = K c * (PotET ref ) (where K c varies with date and actual crop) PotET ref is often estimated by the Penman Monteith equation. See e.g. :

Conceptual Diagram of the Penman Monteith Approach “Aerodynamic Resistance” Wind Speed Crop Height “Canopy Resistance” Stomotal Resistance Leaf Area Index “Surface Energy” Incoming Solar Radiation Outgoing Longwave “Vapor Pressure Deficit” Temperature Relative Humidity Potential Evapotranspiration

Gridded Daily Precipitation and Temperature Records VIC Hydrology Model Daily Time Series of Estimated Reference Crop PotET Schematic Diagram of Simulation Tool for Producing Long Records of PotET

Seasonal Cycle of PotET for a Single Grid Cell in the Snake River Plain

Average July PotET for Alfalfa Reference Crop Potential ET (mm)

Four Delta Method Climate Change Scenarios for the PNW ~ C ~ C Somewhat wetter winters and perhaps somewhat dryer summers

Average July PotET over the Southern Plain Region Current Climate vs. MPI2040 scenario Current Climate MPI2040 PotET (mm/day)

Trends in July Avg PotET over the Southern Plain Region from

Conclusions Long-term gridded temperature and precipitation records can be used to drive hydrologic models to simulate potential ET for a reference crop. Simple experiments in which the temperature is perturbed while other explanatory variables remain about the same suggest that crop water demand ought to be going up over time as the region warms. The long term historic simulations, however, show that the trends are downward over time. One possible explanation for these trends is associated with increasing night time temperatures, which indicate that atmospheric moisture content is systematically increasing. This reduces the vapor pressure deficit and the incoming solar radiation. These results suggest that changes in relative humidity, cloudiness, and wind may play a more dominant role than temperature alone in controlling ET. If so, more sophisticated methods for evaluating the effects of changing climate on these variables will be needed to better assess the potential changes.