Monitoring Agricultural Drought in Australia - Monitoring and Predicting Agricultural Drought: A Global Study - page 378

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of runoff, drainage, soil evaporation, and transpiration are based on inputs
of rainfall and pan evaporation. A daily plant-growth index is calculated
from separate indices representing plant growth response to water stress
(ratio of actual to potential transpiration), air temperature, vapor pres-
sure deficit, solar radiation, and nitrogen availability. At low pasture cover,
plant growth is calculated as a function of the plant growth index, plant
density, and potential regrowth rate. As green cover increases, plant growth
is calculated from a combination of temperature response, transpiration-
efficiency, radiation-efficiency, and nitrogen limitations. For native pasture
simulations, trees compete for water and nitrogen. Pasture biomass is calcu-
lated as the net result of pasture growth, detachment, and intake by grazing
animals.
The GRASP model is calibrated against field data to obtain the main soil
and plant parameters (McKeon et al., 1990). GRASP has been evaluated at
a small plot and paddock scale for tropical pastures in northern Australia
(over 100 locations, < 29° S latitude; figure 29.1; Day et al., 1997) and
temperate pastures in southern Australia (16 locations, > 29° S latitude;
figure 29.1; Tupper et al., 2001). Output from the spatial implementation
of this model has been evaluated through broad-scale field surveys of pas-
ture biomass (e.g., Carter et al., 2000; Hall et al., 2001) and, as described
later in this chapter, against historical drought records.

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Sp atial Implementation of the Models

Rainfall and climate inputs are a generic requirement of all agronomic
models. As the spatial framework runs daily time-step models, rainfall and
climate data are input on individual 5-km grids (surfaces) for each day
and climate element (Jeffrey et al., 2001). The surfaces are automatically
created by interpolating records from individual recording stations, which
are downloaded each day from the national climate database maintained
by the NCC. Although rainfall is interpolated on a daily basis, the monthly
rainfall totals are considered more accurate because they have been man-
ually checked at BoM. Hence the daily rainfall surfaces are corrected such
that they sum to equal the monthly surface as generated from the manually
checked monthly rainfall totals. As a final check, the monthly rainfall sur-
face is visually compared to the equivalent map on BoM's Web site. The
framework also includes historical rainfall data from 1890 onward and
daily climate data (temperature, humidity, and solar radiation) from 1957
onward (from 1970 for evaporation). Daily climate averages are being used
for the 1890-1956 period until archival data from 1890 are added to the
database.
Apart from the rainfall and climate inputs, the GRASP model also re-
quires inputs of tree density, stock numbers, and specific parameters for
different pasture types. These inputs are incorporated in the modeling
framework as separate surfaces. Although important for determining the
absolute amount of pasture production or biomass, these factors do not

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