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Overview

Description and rationale

Source models the use of water by a combination of supply point and water user nodes.  The water user node provides a range of demand models that can be configured to represent irrigation demand. Three different models of irrigation demand have been incorporated into Source to represent the different approaches used in Australia. These three approaches include:

1)    Regression models, used by the Murray Darling Basin Authority (MDBA)

2)    PRIDE Demand model - SRG, used in Victoria

3)   Crop Model 2 Demand model SRG, used in NSW and QLD.

In addition, Melbourne University as part of the eWater CRC, undertook a number of years of research looking at how to improve demand modelling. A prototype irrigation demand model (NGenIrr) was developed as part of this research. The Irrigator demand model was developed by combining the best functionality from the existing models into a common demand model. A key focus was on keeping the model as simple and parsimonious as possible, while not compromising the key functional requirements.

Irrigator operates on a daily basis generating demands and extracting water to meet these demands via the water user and supply nodes. The crop water balance for each crop is represented using the method outlined in FAO Irrigation and Drainage Paper 56, Crop Evapotranspiration – Guidelines for computing crop water requirements (Allen et al, 1998).The model can be applied in both regulated and unregulated systems. Irrigator includes a number of enhancements that have been previous identified by a range of studies into the existing irrigation demand modelling approaches(Ribbons and Podger, Ranchum and Sheedy, Zaman et al, ). Irrigator allows crop based planting decisions which better aligns with economic modelling.

Scale

Irrigator is conceptualised to represent an irrigation district scale rather than an individual irrigator. It only operates on a daily time-step.

Principal developer

This model was developed by eWater Ltd. with technical contributons contributions from Matt Bethune and Geoff Podger and software development by Chris Wilson.

Scientific

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provenance

The Irrigator demand model is based on the existing demand models, including the NOW IQQM Crop Model 2, Victorian Pride Model and the FAO crop water use guidelines (FAO-56). The strengths from each of these demand models have been extracted and included into the Irrigator demand model. In addition, new functional requirements identified by NSW NOW reviews and Victorian studies have been included.

Version

Source V3.2.6

Dependencies

Irrigator is applied through a water user node, which must be connected to at least one supply point node to provide water to satisfy the irrigation demand.

Availability/conditions

Automatically included with Source. 

Structure

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and processes

Background

Irrigator is designed to represent the regulated and opportunistic irrigation requirements of a district.  The irrigation requirements are used by the Water User to generate and place orders and opportunistic requests and to subsequently  extract water from a water source (supply point or water user storage). The Water User will limit orders and extractions  subject to water availability and extraction constraints.

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There are three key processing steps in the Irrigator model.1)   

1. Forecasting requirements – During the Ordering Phase

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1. Current demand – Prior to Flow Phase

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1. Supplying ordered water and managing return flows– During Flow Phase

These three steps are summarised below. Further details on the calculations follow. 

Forecasting requirements

1. The Water User informs the Irrigator of how much water is available and requests from the Irrigator a list of both regulated and opportunistic requirements between now and the maximum travel time. Note maximum travel time is determined as the maximum time to meet order and requests from all of the supply points.
2. The Irrigator model evaluates for each time step between now and the maximum travel time;
   1. Changes in crop area in response to planting decision triggers and available water.
   2. Projected soil depletion for each active crop, using forecasts of potential evapotranspiration and rainfall. This is described below in the section on forecasting depletion.
   3. Calculates both the regulated and opportunistic requirement for each crop based on the volume of water required to keep the soil at respective minimum target depletions or pond levels, including consideration of crop irrigation efficiencies.
   4. Calculates total regulated and opportunistic requirements for the district, by summing individual requirements and factors for escapes.
3. The Irrigator Model returns a list of regulated and opportunistic requirements between now and the maximum travel time to the Water User.

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The soil water balance for each crop is modelled using a single layer moisture store (Equation  1), as outlined in FAO 56 – Chapter 8. Equation 85 from FAO56 represents the water balance of the rootzone.

Equation 

Equation 1

Image Modified

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where:

D_r,i root zone depletion at the end of day i [m],

D_{r, i-1} water content in the root zone at the end of the previous day, i-1 [m],

P_i effective precipitation on day i [m],

I_i irrigation depth on day i [m],

RO_i runoff from the soil surface on day i, including rainfall and irrigation [m],

CR_i capillary rise from the groundwater table on day i [m],

ET_c-adj crop evapotranspiration on day i [m],

DPi DP_i water loss out of the root zone by deep percolation on day i [m].

Soil water depletion

The amount of water in the rootzone is defined in terms of rootzone depletion. Rootzone depletion describes the soil water deficit of the rootzone, relative to field capacity. A rootzone depletion of 0 would mean the soil is at field capacity. Positive depletions correspond to soils drier than field capacity. Negative depletions correspond with soils wetter than field capacity and can be used to represent pond depths for ponded crops.

The user specifies the soil moisture capacity at the district level and this is assumed constant for all crops in the district (Equation  2). The soil moisture capacity is given by the difference in soil moisture content between field capacity and parmenent permanent wilting point. The total available water (TAW) for each crop is a product of the depth of the rootzone (when fully grown) and q Cap (Equation  Equation 3). The depth of the rootzone is specified for each crop. The depth of rootzone is also specified for fallow. While the fallow has no rootzone, this is treated as the depth of soil from which evaporation can occur.

Equation 

Equation 2

Image ModifiedEquation

Equation 3

Image Modified

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where:

TAW the total available soil water in the root zone [m],

_FC the water content at field capacity [m3 m³ m^-3],

 _WP the water content at wilting point [m3 m³ m^-3],

 _Cap  the soil moisture capacity [m3 m³ m^-3],

Z_r the rooting depth [m].

Crop water use occurs at potential rates until soil depletion equals the readily available water (RAW) (Figure 2). RAW is defined as in FAO56 (Equation  4) .The water stress coefficient (K_s) defines crop water use response relative to soil water depletion (Equation  5). K_s decreases linearly from 1 when soil depletion exceeds RAW to 0 at soil depletions levels greater than TAW.  (Figure 2). When Ks K_s equals 0, the crop is assumed to die. When the crop dies, the area is returned to fallow and no more irrigation requirements are generated for this crop. The crop can only be re-established if triggered by a new planting decision.

Readily available water is defined by:

Equation 

Equation 4

Image Modified

p is the average fraction of Total Available Soil Water (TAW) that can be depleted from the rootzone before moisture stress (reduction in ET) occurs [0-1].

For Dr D_r > RAW, Ks K_s is given by:

 

Equation 

Equation 5

Image Modified

Where Ks where:

K_s is a dimensionless transpiration reduction factor dependent on available soil water [0 - 1],

D_r root zone depletion [m],

TAW total available soil water in the root zone [m],

p fraction of TAW that a crop can extract from the root zone without suffering water stress [-].

Figure 5. Relationship between water stress coefficient and soil water depletion.

Crop Evapotranspiration

Crop evapotranspiration is calculated using the single crop coefficient approach described in FAO-56 (Equation  6). The effects of soil water stress on crop ET are calculated by multiplying the crop coefficient by the water stress coefficient (K_s). The  The K_s value is evaluated based on soil water depletion at the start of the time-step.

Equation 

Equation 6

Image Modified

 where:

K_c = Single crop coefficient on growth day i of the crop

K_s = water stress coefficient  describes the effect of water stress on crop transpiration

ET_o = reference crop evapotranspiration

ET_{c adj} = Crop evapotranspiration after adjustment for water stress.

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Effective rain considers that a proportion of rainfall will be interception or lost and not infiltrate. FAO-56 proposes an interception loss of 0.2 * ETo ET_o. This interception loss is included into Irrigation to define effective rain as described in (Equation  7).

Equation 

Equation 7

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Rainfall Runoff

Rainfall runoff occurs when rainfall results in soil moisture exceeding saturation or a maximum target pond level for ponded crops.

Equation 

Equation 8

Image Modified

Wherewhere:

RainfallRunoff = the amount of rainfall runoff depth for cropping area (m)

Target_op = the opportunistic target depletion level for a cropping area (m)

Dr_t-1 = soil water depletion at the beginning of the time step (m)

 the P_e = the amount of effective rainfall (m)

DeepPercolation –  = amount of deep percolation (m)

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The processing of escapes at both a district and crop level is the same. For each escape, the escape volume is firstly calculated (Equation  9), the escape volume is removed from the volume of water supplied (Equation  Equation 10) and then the return volume is evaluated (Equation  Equation 11). Where there are multiple escapes, the escape factors are effectively multiplicative.

Equation 

Equation 9

Image ModifiedEquation

Equation 10

Image ModifiedEquation

Equation 11

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The volume supplied is reduced by the escape volumeNotes

Where

EscapeVolume_n = the escape volume through escape mechanism ‘n’

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