A comparative assessment of AWBM and SimHyd for forested watersheds Bofu Yu1, Zhongli Zhu2 1School of Engineering, Griffith University, Australia 2 School of Geography, Beijing Normal University, China

Background Workshop objective: to apply and evaluate model performance in simulating ‘non-stationary’ hydrologic time series; Climate and streamflow data were provided for 14 watersheds/catchments around the world; Protocol was stipulated for consistency for all modellers; AWBM and SimHyd are by far the most commonly used models for streamflow estimation in Australia; Two smallest of the 14 watersheds/catchments were selected for comparison purposes. Reasons for selecting small watersheds were is that rainfall and climate data are usually much better for smaller watersheds, and the effect of hydrologic lag is minimal.

Two models AWBM SimHyd Model type Conceptual Developer Walter Boughton Francis Chiew Where model developed Brisbane, Australia Melbourne, Australia Latitude (degree) -27.50 -37.78 Time step Daily No. of parameters 8 9 Input data requirements Rainfall and potential evaporation

AWBM

SimHyd

Model comparison With default parameter values for AWBM and SimHyd, and max. retention value of 105mm for the SCS Curve Number method, all models show a non-linear relationship between rain excess and runoff. NB: All the ‘stores’ in AWBM and SimHyd were assumed to be half full initially.

Two watersheds Rimbaud Fernow Country France The U.S. Watershed The Ruisseau du Rimbaud at Collobrieres The Fernow EF River at Watershed 6 Closest major city Nice Pittsburgh Latitude (degree) 44.24 39.07 Area (km2) 1.4 0.2 Period 1967-2006 1959-2009 Land use Forest Cause for non-stationarity Fire in 1990 Clearing 1964-68, re-forestation in 1973 Gross runoff coefficient 60% 41% Solid prec./total prec. <1% 14%

- models developed - watersheds applied

Working Hypotheses None of the 2 models is going to work; Conceptual models would perform better for watersheds with a higher runoff coefficient and (for these 2) with less snow fall

Two watersheds Rimbaud Fernow Country France The U.S. Watershed The Ruisseau du Rimbaud at Collobrieres The Fernow EF River at Watershed 6 Closest major city Nice Pittsburgh Latitude (degree) 44.24 39.07 Area (km2) 1.4 0.2 Period 1967-2006 1959-2009 Land use Forest Cause for non-stationarity Fire in 1990 Clearing 1964-68, re-forestation in 1973 Gross runoff coefficient 60% 41% Solid prec./total prec. <1% 14%

Model comparison in terms of the N-S coefficient of efficiency for 30 calibration-validation combinations AWBM SimHyd Fernow, the US 0.43±0.11 0.40±0.12 Rimbaud, France 0.79±0.07 0.77±0.09 There is a much larger difference between watersheds than between models!!

Model performance in terms of NSE between 2 watersheds Histograms of the Nash-Sutcliffe coefficient of efficiency for 30 calibration-validation combinations (level 1 & 2 requirements) For the Rimabaud, no NSE is less than 0.6, for Fernow none is above 0.6.

Rimbaud

Rimbaud

Fernow

Fernow

Observed hydrologic changes at Fernow Variable Period No. of years Rate (mm.yr-1) p-value Precipitation 1959-1986 28 +6.7±3.1 0.04 Streamflow +11.4±2.5 <0.01 1986-2009 24 -2.2±5.5 0.69 -10.1±4.6 1959-2009 51 1.3±1.5 0.40

Fernow

Observed hydrologic changes at Rimbaud Variable Period No. of years Rate (mm.yr-1) p-value Precipitation 1968-2006 39 -11.3±4.3 0.01 Streamflow -14.8±4.2 <0.01 Both precipitation and streamflow have significantly decreased over the 39 year period.

Rimbaud

Conclusions (what we have learnt from this exercise) Difference in model performance is small if calibration method is used consistently; Source of non-stationarity is important; Conceptual models tested are adequate in describing the effect of changes in precipitation, not the effect of changes in vegetation; Changes to streamflow are greater than those in precipitation for these watersheds.

Conclusions .. (what we have learnt from this exercise) The effect of the fire in the Rimbaud watershed is secondary in the context of the overwhelming decreasing trend in precipitation and streamflow over the 39 years; The 2 conceptual models tested did not perform well for the Fernow (W6) watershed because of the significant increase in streamflow early in the study period, and the subsequent significant decrease in streamflow from 1986. And it snows a lot over there.

Thank You!