Published on January 20, 2008
THE EFFECT OF SOIL DATASETS ON OFFLINE CABLE SOIL MOISTURE SIMULATIONS IN THE MURRAY DARLING BASIN Adam Smith1,2 and Huqiang Zhang2 1 Water Division, Bureau of Meteorology, Australia 2 Centre for Australian Weather and Climate Research (CAWCR), a partnership between the Bureau of Meteorology and CSIRO, Australia Contact details: [email protected]: DATA The soil moisture monitoring sites used to evaluate CABLE are located in the Murrumbidgee Catchment (Fig. 3). This catchment is part of the Murray Darling Basin, contains the Australian Capital Territory (ACT) and has land use varying from the Snowy Mountain Hydro-Electric Scheme at high elevation in the south east to the Colleambally and Murrumbidgee Irrigation Areas in the planes of the west. RESULTS THE EFFECT OF SOIL DATASETS ON OFFLINE CABLE SOIL MOISTURE SIMULATIONS IN THE MURRAY DARLING BASIN Adam Smith1,2 and Huqiang Zhang2 1 Water Division, Bureau of Meteorology, Australia 2 Centre for Australian Weather and Climate Research (CAWCR), a partnership between the Bureau of Meteorology and CSIRO, Australia Contact details: [email protected] MOTIVATION The CSIRO Atmospheric Biosphere Land Exchange (CABLE) model is the land surface model (LSM) used in the Australian Community Climate Earth System Simulator (ACCESS). ACCESS is to be used by both the Australian Bureau of Meteorology and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) for operational numerical weather prediction (NWP) and climate studies (both regional and global). Currently VB95 (Viterbo and Beljaars, 1995), an older version of the European Centre for Medium-Range Weather Forecasting (ECMWF) LSM the Tiled ECMWF Scheme for Surface Exchanges over Land (TESSEL) (Van den Hurk et al. 2000), is used operationally at the Bureau of Meteorology for NWP. Richter et al. (2004) assessed the sensitivity of VB95 to soil and vegetation parameters via offline simulations at 10 locations in the temperate Murrumbidgee River Catchment (NSW, Australia). Similarly, using observed soil moisture data at the same 10 locations in the Murrumbidgee Catchment (located within the Murray Darling Basin) this work assesses the impact of replacing the default CABLE soil parameters (taken from the Zobler (1986) global 1° resolution soil map) with parameters derived from the 1:2,000,000 scale Digital Atlas of Australian Soils. CABLE The CSIRO Atmosphere Biosphere Land Exchange (CABLE) land surface model is a 3rd generation model in the classification of Pitman (2003). CABLE has 6 layers for solution of soil moisture and heat using Richard’s and the heat equation respectively (with layer depths from the surface of 2.2., 8, 23.4, 64.3, 172.8 and 360 cm), and a 3 layer snowpack model that solves for albedo at the surface, as well as temperature, density and thickness of each layer, additionally permafrost (frozen soil) is modelled (Kowalczyk et al. 2006). CABLE has a single (above ground) two “big” leaf (shaded and sun-lit) canopy for calculation of stomatal conductance, photosynthesis and leaf temperature (for each “big” leaf) (Wang & Leuning 1998; Wang 2000), and a turbulence model to calculate within canopy air temperature and humidity (Raupach et al. 1997). Virginia Park MURRUMBIDGEE RIVER CATCHMENT Figure 3: Topography of the Murrumbidgee River Catchment. Yellow dots are soil moisture monitoring sites. Forcing data is needed to run a land surface model offline (i.e. uncoupled from its parent atmospheric model). This data was derived from 15 automatic weather station (AWS) sites. The AWS data is complemented by manual observations of cloud cover and sunshine hours, and in-situ and satellite observations of radiation (Siriwardena et al. 2003). This data set begins in January 2000 and is updated every six months; for this study January 2000 to December 2006 is used. Other data required by CABLE include Leaf Area Index (LAI), dominant vegetation and soil type. Mean monthly LAI is calculated from remotely sensed 0.05° monthly average woody and herbaceous fractional cover, for the period 1981-1994 (Lu et al. 2003). Dominant vegetation and soil types are given by the (CABLE default) global 2° resolution maps of 13 vegetation (Potter et al. 1993) and 9 soil classes (Zobler 1986). Both of these datasets have been aggregated from the 1° originals. The Zobler map is based on the FAO/UNESCO Soil Map of the World. CONCLUSIONS Overall the performance of CABLE in simulating soil moisture (with the default Zobler (1986) soil parameters) is quite satisfactory. Replacing the default parameters with ones derived from the Australian Atlas of Soils resulted in degraded model performance at the majority of sites. This is due to unreliable B horizon parameters being used to create a depth weighted average, it is believed that using only the A horizon parameters will result in considerable improvement of the Atlas results. The default predictions matched the variability in observed soil moisture quite well (particularly for the surface 0-7 and 0-30 cm soil moisture) however a bias was clear in the deeper soil moisture predictions (30-60 and 60-90cm) due to the (duplex) soil profile being represented by one soil type in the model. In comparison to the default simulations the soil moisture predictions from the Atlas reduced the deeper soil moisture (30-60 and 60-90 cm) bias (due to the B horizon being included in the Atlas parameter estimates), however this improvement was swamped by the woeful simulation of the surface (0-7 and 0-30 cm) soil moisture (due to the b parameter being far too high). At this stage it is recommended to continue using the default (Zobler 1986) global soil dataset for the higher spatial resolution requirements of NWP and regional climate simulations. Figure 2: Zobler (1986) 1 degree resolution soil types. Figure 1: Murrumbidgee Catchment spatial datasets. a) Elevation b) Precipitation c) Soil d) Geology e) Climate f) Areal PET CABLE simulates soil moisture in the top 90 cm fairly well. In particular the temporal dynamics in the 0-7 and 0-30 cm observations are well represented at most sites, however the wilting point (minimum soil moisture) in CABLE simulations is consistently too high (see top two panels of Figure 4). There is a bias in all CABLE simulations of the (lower) 30-60 and 60-90cm soil moisture (Figure 4 is one example) due to the soils of the Murrumbidgee sites being a duplex type with a clear transition between an A and a B horizon at a depth of about 30 cm. Currently CABLE represents the entire soil profile as a single soil type, due in part to the soil moisture based formulation of Richard’s Equation used by the model to simulate water flux. Replacing the default soil parameters with those derived from the Atlas did not result in any model improvement, in fact at the majority of sites (6 of 10) model performance was severely degraded (Figure 5 is one example). This is due to the parameters used for the Atlas runs being created as a depth weighted average of the A and B horizon parameters. This gave most weighting to the B horizon, because the A horizon has a depth of 20 or 30 cm, whereas the B horizon depth ranges from 50 to 110 cm. Figure 4: Kyeamba soil moisture using the default Zobler (1986) soil dataset. Green line is predicted and red is observed. Top panel is 0-7 cm, next 0-30 cm, third 30-60 cm and bottom is 60-90 cm soil moisture. Figure 5: Same as Figure 2 but for the Australian Atlas soil dataset.