SOIL MOISTURE MODELLING AS A FLEXIBLE TOOL FOR SIMULATING
THE IMPACTS OF WASTEWATER IRRIGATION SCHEMES
Woodhouse, C. D.,1 Bradley, D.1 Pattle, A.1
1
Pattle Delamore Partners Ltd
Aims and Introduction
Soil Moisture Deficit (SMD) modelling has been traditionally used in farming to estimate the water
required for cultivation of specific crops to improve water use efficiency. PDP have altered this
modelling technique for use in wastewater irrigation applications. This revised modelling
technique is simple and can be easily modified to suit project specific requirements. This
modelling technique can be used to:
-
Optimise the volume of wastewater irrigated to land, by allowing for wastewater storage;
Predict the potential environmental impacts of a given wastewater irrigation depth; and
Provide inputs to other models, such as recharge in a groundwater flow model.
Methods
Model calculations are based on a one dimensional soil moisture water balance model.
Precipitation and irrigation are the inflows, with evapotranspiration, runoff, and percolation to
groundwater (recharge) the outflows. The revised model is based on a “deficit irrigation regime”,
which ensures that there cannot be any direct runoff of wastewater effluent, and that all irrigated
effluent must pass through the soil profile. This infiltration rate should be based on field
measurements, typically obtained by using a double ring infiltrometer. The deficit irrigation
regime ensures that runoff of effluent cannot occur and means that irrigated water must pass
through the soil profile, thus allowing vegetation and the soil itself to remove nutrients.
Figure 1: Conceptualisation
soil moisture model.
of the soil zone used in the
SMD modelling uses
evapotranspiration values
data can be readily
databases.
Information
crop characteristics can
values
based
on
and crop type. For more
analysis
of
soil
performed.
The model
moisture and the recharge to groundwater at daily intervals.
rainfall
and
at a daily time step. This
obtained from climate
regarding the soil and
be sourced from literature
observations of the soil
reliable results further
characteristics can be
then
calculates
soil
More complex models are available which quantify similar processes, however the small amount
of data required by this SMD modelling technique, and the simplicity with which it operates,
means that minimal processing power is required. This simplicity means that it can be easily
modified to suit specific applications or project requirements.
Applications
SMD modelling has been used successfully in predicting the impacts and improving efficiency of
wastewater irrigation schemes across New Zealand. At a residential development in northern
Auckland, this model was used to predict the amount of water storage required to dispose of a set
amount of wastewater to a specified land area, with no leakage into the groundwater.
This technique can also be used to determine the additional recharge to groundwater caused by
wastewater irrigation. An example from the Wairarapa highlights the issues and benefits of this
approach and how this model can be used as a tool for infrastructure planning.
The modelling can also be utilised as an input to larger groundwater models. Using modelled
recharge to groundwater is better practice than assuming recharge rates based on groundwater
head calibration data alone. This approach was used to the north of Auckland, where the model
was used to predict groundwater inflows both with and without a wastewater irrigation scheme, in
order to represent the impact of the scheme on recharge rates.
A challenge of SMD modelling is the uncertainties which can arise from the use of non-crop
plants, which vary widely in evapotranspiration rates. In particular, some of the wastewater
irrigation schemes that this model was used to represent encompass native vegetation or mixed
forest, making evapotranspiration rates difficult to accurately predict.
A modified version of the model was used in the Manawatu District to optimise the volume of
irrigation using wastewater storage on pasture with underlying silt and clay. Wastewater storage
was incorporated into the model to maximise wastewater irrigation, and the model was linked to
groundwater mounding calculations to ensure an unacceptable degree of mounding did not
occur.