Hydrologic Modeling in GCIP and GAPP Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington GEWEX Americas Prediction Project PIs Meeting Seattle July 21-23, 2003

Perspectives on the land surface from above and below (or, what is the difference between a land surface scheme and a hydrologic model? Hydrologic Model: Predict Q (and perhaps other variables related to surface and subsurface moisture) given P and Ep Land surface model: Predict partitioning of Rn given downward solar and longwave radiation

General situation c. 1990 Hydrologic model: Typically compute Ep given T (and perhaps other variables) but often w/o explicit representation of vegetation. The calibrate for parameters related to soil properties. Often implicit assumption (if needed) is that Ts = Ta L/S model: Given vegetation and associated parameters, iterate for Ts by closing water and energy balances simultaneously. Runoff is essentially residual of P, E, and storage change.

Trends last 10-15 years Hydrology models have been driven toward explicit vegetation representations by the need to predict land cover change implications, among other factors L/S models have been driven to do a better job wrt land surface hydrologic variables via realization that errors in water cycle representation affect surface energy balance, i.e., errors in runoff  errors in ET  errors in energy partitioning (and timing)

Visual courtesy Sean Carey, U. Saskatchewan

So, is there a need for hydrologic models that are not also L/S schemes, and vice-versa? Note challenges in hydrologic prediction don’t necessarily require balancing of energy Is there a rationale for or against eventually doing all hydrologic prediction with fully coupled models (bias issues?) Can we show value in terms of weather and climate prediction of more accurately predicting land surface hydrologic dynamics

Some other challenges for GAPP hydrology Can we document the evolution of hydrologic forecast skill? Do we understand the potential skill in S/I hydrologic forecasting How do we improve estimates of hydrologic initial conditions? What is the potential worth of hydrologic forecasts What are the hydrologic sensitivities to drought?

1) Documentation (or lack thereof) of hydrologic forecast skill Visual courtesy Tom Pagano, NRCS

2) Potential skill of hydrologic forecasts 1.5 4.5 7.5 10.5 13.5 Lead, months Source: Maurer and Lettenmaier, 2003

3) If S/I hydrologic predictability is mostly in the initial conditions (vs climate forecast skill), what are we doing about improving our estimates?

4) What is the economic worth of S/I hydrologic forecasts?

Increasing marketing emphasis on late summer and fall hydropower Value of long-range hydrologic forecasts for hydropower production in the Columbia River basin (from Hamlet and Lettenmaier, 1999)

5) Hydrologic sensitivities to drought: Rio Bravo/Rio Conchos River annual flows, 1954-2001 Note: does not include some upstream inflows due to lack of extended data 4978Mm3 (1955 to 1992 avg) 2542 Mm3 (1993 to 2001 avg)