Evaluation of TRMM satellite precipitation product in hydrologic simulations of La Plata Basin Fengge Su 1, Yang Hong 2, and Dennis P. Lettenmaier 1 1 University of Washington, Seattle, WA 2 NASA Goddard Space Flight Center, Greenbelt, MD

1. Background The objective of this study is to evaluate the TRMM-based precipitation estimates, and their utility in hydrologic predictions. Precipitation is the primary atmospheric input to hydrology models. Satellite-based precipitation estimates show potential attractions for hydrologic predictions. High resolution TRMM-based products are available to research community.

The TRMM product evaluated in this study is the TRMM Multi-satellite Precipitation Analysis (TMPA) research product Version 6 3B42. It combines precipitation estimates from multiple satellites, as well as gauge analyses, where available, at a 3-hour time step and 0.25° degree spatial resolution. The data set covers the latitude band 50°N-S for the period 1998 to the delayed present. 2. Data sets and methodology Huffman et al., 2006, The TRMM Multi-satellite Precipitation Analysis (TMPA): Quasi- Global, Multi-Year, Combined-Sensor Precipitation Estimates at Fine Scales (to appear in Journal of Hydrometeorology)

Rain gauge distribution in 1998 (846) Rain Gauge distribution in 1999 (676) La Plata Basin and Gauged data ( ) from NCAR and GDCN (Global Daily Climatology Network) station data Uruguay Parana Paraguay

Methodology 1.Basin average precipitation estimates from the TRMM Satellite precipitation product (Version 6 3B42) and gauges are compared with each other at both daily and monthly time scales. 2.The Variable Infiltration Capacity (VIC) land surface hydrology model is forced by daily 3B42 and gauged precipitation over several subbasions of La Plata. The simulated streamflow is compared with each other and with available observed streamflow.

3. Precipitation evaluation Scattergrams of daily basin-averaged precipitation estimated from gauged and TRMM data. Rrmse=60%Rrmse=13% Rrmse=134%Rrmse=15% Rrmse=44%

Scattergrams of monthly basin- averaged precipitation estimated from gauged and TRMM data. Monthly Basin-averaged Precipitation

Daily time series of precipitation for basin Gauged TRMM Month mm/dy

Monthly time series of precipitation from gauged and TRMM 3B42 data for five subbasins ( ). Basin Relative Error (%) Average

FBI can indicate whether there is a tendency to underestimate (FBI 1) rainy events. POD gives a proportion of observed rain events successfully estimated by the TRMM data. FAR provides a measure of the 3B42’s tendency to estimate rain where none was observed. Statistical evaluation over different subbasins Frequency Bias index Probability of Detection False Alarms Ratio

4. Hydrological Modelling The VIC (Liang et al, 1994;1996) model is a grid-based land surface scheme designed both for inclusion in GCMs, and for use as a stand-alone macro- scale hydrological model. Model features: multiple vegetation classes in each cell; energy and water budget closure at each time step; subgrid infiltration and runoff variability; and non-linear baseflow generation.

Observed Simulated with TRMM Prce. Simulated with gauged Prce. Nash-Sutcliffe efficiency (Ef) Relative error (Er) Daily streamflow for basin , Uruguay at Paso de Los Libres (Area: 189, 300 km 2 ) Green VS. Black: Ef = 0.85, Er = 2% Red VS. Black: Ef = 0.48, Er = 12% Red VS. Green: Ef = 0.54, Er = 16% m 3 /s m 3 /s m 3 /s

Daily streamflow for basin 6301, 6598, and 6682 Observed Simulated with TRMM Prce. Simulated with gauged Prce. Nash-Sutcliffe efficiency (Ef) Relative error (Er) Red VS. Green: Ef = 0.73, Er = 4% Red VS. Green: Ef = 0.73, Er = -1% Red VS. Green: Ef = 0.72, Er = 18% (a) 6301, Parana at Jupia (Area: 478,000km 2 ) (b) 6598, Iguazu at Estreito (Area: 63,236 km 2 ) (c) 6682, Paraguay at Ladario (Area: 459,990 km 2 ) m 3 /s m 3 /s m 3 /s

Monthly streamflow for the four subbasins Observed Simulated with TRMM Prce. Simulated with gauged Prce. m 3 /s m 3 /s m 3 /s m 3 /s (a) 3802, Uruguay at Paso de Los Libres (b) 6301, Parana at Jupia (c) 6598, Iguazu at Estreito (d) 6682, Paraguay at Ladario

The Version 6 3B42 provides good performance at monthly time scale. The daily estimates show a good performance for the low and medium precipitation thresholds. For higher thresholds, satellite estimates tend to overestimate the frequency of rain events, and show low POD and high FAR. The 3B42-driven model results show a good ability in reproducing the timing of floods, and in representing the low peak flows, but tend to overestimate the high-flow peaks. Despite the errors in the satellite-based precipitation estimates, this study provides encouraging results of using the TRMM 3B42 product in basin-scale hydrology predictions. 5. Conclusion