Page Comparison

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 include the

IQQM crop model. The IQQM crop model operates on a daily basis generating demands and extracting water to meet these demands via the water user and supply nodes. The model can be applied in both regulated and unregulated systems.

The IQQM crop model maintains a soil moisture store for each irrigated crop as well as the fallow land with the combined areas of these stores equal to the maximum irrigable area multiplied by a development factor. The fluxes associated with the crop stores are effective rainfall, irrigation, runoff, evapotranspiration and seepage. The fluxes associated with the upper fallow store are rainfall, runoff, evaporation and infiltration and with the lower fallow store are infiltration and seepage. Irrigation water is supplied by the water user and total return flow and seepage are returned to the water user. The fate of the return flow and seepage is dependent on how the water user is configured to manage this water. Return flow can be sent to the water user storage or returned to a confluence node. Seepage can be input to a surface water ground water interaction model. Alternatively these fluxes can be treated as a loss.storage or returned to a confluence node. Seepage can be input to a surface water ground water interaction model. Alternatively these fluxes can be treated as a loss.

where:

M_t is the soil moisture depth at the end of the current time-step (m)

M_t-1 is the soil moisture depth at the end of the previous time-step (m)

P_e is the effective rainfall (m/s)

E_a is the actual evapotranspiration (m/s)

S is the seepage (m/s)

I is the irrigation applied per unit area (m)

dt is the model time-step (s)

The IQQM crop model has two planting decision dates each year. These are for the primary and secondary crops. Primary crops are the crops that are predominantly supplied by irrigation. In the Murray-Darling basin this is usually summer crops. The secondary crops are largely rain fed and use any remaining irrigation to maintain soil moisture. In the Murray-Darling Basin these are usually winter crops.

On a decision date the user can optionally specify a farmer’s risk function that decides on the area of primary or secondary crops to plant. The function takes into consideration available resource, expected resource and antecedent conditions to determine the area to plant. Note that only a decision on area is made at the planting decision date and the crop area only becomes active on the specified crop start date.

Perennial crops such as orchards and trees grow all year and are considered as primary crops in the model. The area of these crops will change on the planting decision date.

Scale

IQQM crop model is applied at a point scale and operates on a daily time-step.

Principal developer

NSW Office of Water

Scientific

provenance

The IQQM crop model is based on a crop model developed by Lyall & Macoun Consulting Engineers (1986) for the WARAS Lachlan daily model. The model has been substantially modified by the former NSW Department of Land and Water Conservation to accommodate the different irrigation conditions in other NSW regions and internationally (DIPNR, 2004). The IQQM Fortran code was rewritten to fit within the water user structure of Source in C# by New South Wales Office of Water and eWater.

The crop factors used to determine actual evapotranspiration are based on the

FAO56 (Allen, et al, 1998) approach. For further information on how this approach is implemented in Source see the Irrigation Demand Model Crop Factors SRG entry. Source also offers sufficient flexibility to apply daily crop factors if they are known for a particular crop based on a different source or method.

Version

Source V3.02

Dependencies

The IQQM Crop Model 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

and processes

The IQQM Crop Model is a daily soil moisture accounting demand model available for use within a Water User node. It generates water requirements for irrigated crops. Crop water requirements are used to generate demands in regulated and unregulated systems, orders and opportunistic requests within regulated systems and to extract water from a water source, subject to water availability and extraction constraints. Note orders are supplied from storages while request are allocated at off allocation nodes. Water is extracted from an optional water user storage and at Supply Point nodes and is directed to the associated water user node to be distributed to the demand model as a first priority.

The schematic of the irrigable area is shown in Figure

1 and the conceptualisation of the farm supply system is shown in Figure

2. A description of the parameters used by the model is in the

Importing data section

of the User Guide.

Figure

2. Conceptualisation of the farm supply system assumptions and constraints

Image Modified
In developing the original crop model, Lyall & Macoun Consulting Engineers (1986) conducted some numerical tests on 1000 conceptual farms with random distributions of soil moisture and evapotranspiration. During these tests it was assumed that each farm was irrigated when the soil moisture reached zero ie. fully depleted. The study found that irrigation recommenced some time after rainfall and that the depletion curve followed a damped oscillating pattern around half the soil moisture capacity. This is shown (typically) in Figure

3 for a period following saturating rain. For simplicity of modelling, the oscillating curve is replaced by two straight lines as shown in Figure

5.

Figure

3. Irrigation scheduling and adopted soil moisture target curve (a)

Image Modified

Image Modified

Image Modified

The following covers aspects of the theory relevant to IQQM crop model including:

• Area planting decision
• Irrigation demand and opportunistic requirements
• Irrigation application

The workflow between the IQQM crop model and the Water User for each model time-step is as follows:

• When the first supply node is reached in the ordering phase the IQQM crop model checks if it is a planting decision date. If so IQQM determines the area of crops to plant for the respective decision date.
• The IQQM crop model determines if it is the start date for a crop. If so it allocates a soil moisture store from the fallow area based on the area decided on the planting date for the season associated with the crop.
• Based on the soil moisture state for each crop the IQQM crop model calculates today’s and forecast demands and opportunistic requirements (normally these are the same number) up to the maximum delivery time of supply storages and off allocation nodes respectively. Note in unregulated systems this is just today’s requirement. This is subsequently passed to the water user node.
• The water user node allocates the calculated demands and opportunistic requirements to supply points and the optional water user storage subject to supply limitations and licences in regulated systems.
• When the first supply point node (noting that a water user node can be connected to multiple supply point nodes) is reached in the flow phase the water user gathers all of the available water from all supply points and the optional water user storage subject to today’s demand and opportunistic requirement determined in the third step.
• The IQQM crop model applies the available water to the crops.
• The IQQM crop model then determines and passes back any seepage and return flow to the water user node.
• If the water user node is connected to a groundwater model then the seepage is input into the groundwater model.
• If the water user node has a storage then a proportion of the return water is put into the storage. Separate return proportions can be specified for water generated from rainfall derived runoff and irrigation applications. 
• If the water user node is connected to a return confluence any remaining return water or spills from the water user storage are passed to the confluence node.

Forecast irrigation demands and opportunistic requirements are estimated up to the maximum delivery time from the furthest accessible storage and the delivery time from the furthest of the next upstream off allocation sharing nodes respectively. The forecast demand is based on an estimate of the volume required to maintain the soil moisture at a target state or minimum pond depth (for ponded crops such as rice) assuming no rainfall and a forecast evaporation rate. In most cases the opportunistic requirement is equal to the demand. In cases where a crop tolerance is specified (further discussed in a later section on Projected demands and requirements) the opportunistic requirement will be assumed to meet the soil moisture target plus the crop tolerance. Note opportunistic requirements are met from overbank flows and off allocation sharing where specified.

Area

planting decision

The area to be planted by the IQQM crop model may be fixed or optionally determined at the start of a primary or secondary irrigation season based on a farmer’s risk function. The area to plant is determined at the user defined decision date for the primary and secondary irrigation seasons. In the IQQM crop model, farmer’s risk can be modelled in two different ways; an area based approach and a volume based approach. An overview of these follows and they are described in more detail in separate sections on the Area Method and the Volume Method below.

The area based approach is based on a direct conversion rate (in ML/ha) from the farmer’s available resources to the planted area. The farmer’s available resources comprise water in the water user storage and any water available in account balances. The risk is influenced by the slope of the risk line relative to the actual water requirement of the crop. For example if a cotton crop requires 8 ML/ha and the slope is 6 ML/ha then the farmer is taking risk on the remainder of the water becoming available post decision. This approach can be applied in both regulated and unregulated systems.

The volume based approach uses the risk function to calculate the expected available water resource during the season. Based on the model’s calculation of crop requirements and the expected rainfall in the growing season, the expected available resource is then converted into an area to plant. This approach is relevant for regulated river systems that have allocation systems. It is based on the assumption that there will be an increase in allocation during the season. The increase effectively allows for additional inflows to the supply reservoirs that are expected to become available during the season.

For both of these types of risk functions, the value of the risk function depends on the antecedent climatic conditions. For example, in a wet year, the farmers may take more risk to recognise the higher probability of improved resources throughout the year. The modeller can define risk functions for dry, average and wet conditions. The final function used by Source is based on an interpolation between these curves based on current antecedent conditions. The

index for antecedent conditions is calculated as follows;

• the ratio of the six monthly sum of a time series data (typically inflows) to the median prior 6-monthly inflows for the volume based function (Figure

• 6)

• , and
• 2 x the ratio of the fallow soil moisture to the fallow soil moisture capacity the day before the planting decision date for the area based function.

If there is more than one crop type within a season, the area to be planted is divided between the crops based on the user defined proportion values. Although the planting decision is made on a particular date the crop area does not become allocated until the start date for that particular crop.

Risk

function - Area

method

The area to be planted is determined based on:

• available water resources, which is the water available in the water user storage plus the sum of all the account balances;
• an index of antecedent conditions using fallow soil moisture for the day before the planting decision date; and
• a relationship between available water and crop area with different relationships applying to wet, average and dry conditions. The index is used to determine the final relationship used.

The user defined risk function parameters are used in the following calculation:

where:

AP is the area to plant in the irrigation season (

m²)

AW is the available resource (

m³)

m_i is the slope of index curve for the irrigation season (m^-1)

b_i is the intercept of index curve for the irrigation season (

m²)

i is the curve identifier

The calculation in Equation

2 may be constrained based on maximum seasonal planting areas and also based on minimum and maximum parameters in the risk function (refer to Limits to Area Planted).

The slope and intercept of the farmers risk function can be derived by plotting historic data on available resources for each water year versus the area planted for the irrigation season (separate functions can be derived for wet, dry and average conditions if required and data availability permits) This method does not take into consideration other factors that may affect planting decisions such as changes in water sharing rules, water use accounting rules and external factors such as terms of trade/ commodity prices. If scenarios are to be modelled which entail a change in any of these factors, then the function should be re-derived.

If the modeller wants to define the risk function so that some of the water user’s account balance is reserved for the secondary crop then this can only be achieved indirectly by defining a function which plants less area given an available resource. For example if x ML needs to be reserved for the secondary crops then xslope should be subtracted from the intercept for each of the curves for the primary crop. Logically the intercept should not be less than zero.

The choice of slope and intercept values to use depends on the current index value which evaluates how wet, average or dry the conditions are using fallow soil moisture calculated at the end of the previous time-step (ie. the day before the planting decision date):

where:

i is the index used to interpolate between risk curves

FUM_t-1 is the fallow upper soil moisture at the end of the previous time-step (m)

FLM_t-1 is the fallow lower soil moisture at the end of the previous time-step (m)

FUMC is the fallow upper soil moisture capacity (m)

FLMC is the fallow lower soil moisture capacity (m)

If the calculated index ratio falls between two specified curve index values, the area will be calculated based on interpolation between those index results (Equation

4) The lower line and the upper line sets the limits of the area to be irrigated ie. values are not extrapolated. For example, if the index value is greater than the wet relationship index value then the wet index slope and intercept is used.

where:

AP is the area to plant in the irrigation season (

m²)

AW is the available resource (

m³)

i is calculated in Equation

3

m_u is the slope of the upper curve

iu for the irrigation season(m^-1)

b_u is the intercept of the upper curve

iu for the irrigation season (

m²)

i_u is the index of the upper curve for the irrigation season

m_l is the slope of the lower curve

iu for the irrigation season (m^-1)

b_l is the intercept of the lower curve

iu for the irrigation season (

m²)

i_l is the index of the lower curve for the irrigation season

AL_max the specified maximum area for the growing season (

m²)

Risk

function - Volume

method

The area planted is determined based on:

• a user defined risk function which calculates the expected available water resources based on the current available water resource;
• an index of antecedent conditions based on the user-specified inflow data;
• model calculated average total crop water requirements for a irrigation season; and

• expected usable rainfall in the irrigation season.

The user defined risk function parameters are used in the following calculation (see Figure

6):

where:

EAW is the expected available resource (

m³)

AW is the available resource (

m³)

m_i is the slope of the index curve i for the irrigation season

^b_i is the intercept of the index curve i for the irrigation season (

m³)

i is the curve identifier

The planted area is then calculated based on the expected available resource, the estimated crop water requirement and the expected rainfall in the growing season, it is constrained by maximum area limits for the farm and the season.

where:

AP is the area to plant in the irrigation season s (

m²)

EAW is the expected available resource (

m³)

CWR_tot(s) is the total crop water depth (m) requirement for the planted area decision season s (m)

EP(s) is the expected precipitation depth in the planted area decision season s (m)

s is the planted area decision season

Equation

6 demonstrates that the model calculates the crop demand in this risk method based on the average crop evapotranspiration with specified allowance for the minimum or expected rainfall that will occur during the growing season. If the user defines a daily pattern of average evaporation then this is used otherwise a daily pattern is generated based on the input evaporation data.

The total crop water depth requirements are determined during model initialisation using the following formula:

where:

CWR_tot(s) is the total crop water requirement for season s (m)

ncrop(s) is the number of crops in season s

ng(c) is the number of growing days for crop c

K_c(c,j) is the crop factor for crop c on day j

E₀(j) is the forecast potential evapotranspiration rate on day j (m/s)

dt is the model time-step (s)

P_c(c) is the proportion of area of crop c

E_c(c) is the irrigation efficiency of crop c

s is the planted area decision season

The choice of slope and intercept values to use depends on the current index value which evaluates how wet, average or dry the conditions are (see Figure

6) The volume based risk function uses inflow index based on the supplied time series data:

where:

i is the index used to interpolate between risk curves

TS(t-j+1) is the time series inflow at time t-j+1 (

m³/s)

TSM(m) is the specified median of the previous 183 days of inflow for all years in the time series

m is the index month

If the calculated index ratio falls in between the two specified index values, the expected volume will be calculated based on interpolation between those index results (Equation

4) The lower line and the upper line set the limits of the expected volume ie. the values are not extrapolated. For example, if the index value is greater than the wet relationship index value then the wet index slope and intercept are used.

The total area planted within a water year is limited by the following:

• Total crop area is limited to the Maximum Irrigable Area (IA_max=A_totdf) which is the total area of the farm multiplied by the development factor.
• Maximum planting area in the respective primary or secondary season. Note neither of these should exceed the maximum irrigable area (A_max(s)=min(IA_max,A_max(s))).
• For the volume method a minimum area is specified (Equation

• 11)
• For the area method the risk function can be limited by two minimum areas.
• A user-specified area threshold to make a decision to plant or not in the current irrigation season. If the calculated area is less than this threshold, then there is no area to be planted (Equation 212).
• Minimum area to plant based on the risk function. If the calculated area is less than this value and the index is above the average value, then plant this area (ie. this can be used to simulate farmers taking more risk in this situation) This parameter only applies when using the area based risk function.

Equation 10

Image Modified

Volume based

Equation 11

Image Modified

Area based

where:

AP is the area to plant in the irrigation season (

m²)

A_max(s) is maximum planted area for season s (

m²)

i is the index used to interpolate between risk curves

i_avg is the index value of the average risk curve

Volume based

A_min(s) is the user-specified minimum area to plant for season s (

m²)

Area based

A_min(s) is the user-specified minimum area threshold to plant crops for season s (

m²)

A_limit(s) is the user-specified minimum area to plant for season s (

m²)

Irrigation demand and opportunistic requirements

The irrigation demand and opportunistic requirements are determined each day and for regulated systems they are predicted for a defined time into the future. In projecting the soil moisture conditions the model takes into consideration evapotranspiration, seepage and water that has been previously ordered. A demand for irrigation is generated when the projected soil moisture level is less than the target soil moisture level. Orders are generated to satisfy this irrigation demand with consideration given to delivery time from the regulated water source to the extraction node.

The calculation of the irrigation demand requires a calculation of the estimated crop evapotranspiration and subsequently the estimated soil moisture shortfall. The following two sections provide some background on these components of the model.

Crop evapotranspiration (

E_c)

Crop evapotranspiration is determined by multiplying potential reference crop evapotranspiration by a crop factor (K_c) The potential evapotranspiration is a time series input that may be calculated directly by the Penman-Monteith or equivalent method. Alternatively pan evaporation data may be modified by a pan factor (K_pan) to convert to potential reference crop evapotranspiration.

The method for defining crop factors is based on

FAO56 (Allen, et al, 1998), which represents the growth period of each crop by four developmental stages defined as curves. The four stages are:

initial stage

also called the "establishment stage", this stage occurs between planting and when there is approximately 10% ground cover. Water requirements are low and constant.

crop development stage

also called the "vegetative stage", this stage has a rapid increase in the amount of water required by the crop until there is generally about 70%-80% ground cover and maximum rooting depth is achieved.

mid-season stage

also called the "flowering stage", this stage requires the maximum amount of water in the life of the crop.

late stage

also called the "yield" and/or "ripening stage", the water requirement is for crop ripening and reaching maturity.

Crop factors for each modelled time-step are interpolated using co-ordinates of the curve or alternatively entered as a daily pattern. Note crop factors are entered based on growing days and are offset by the start date of the crop.

Setting the crop area and soil moisture

IQQM Crop Model has individual single layer soil moisture stores for each crop type. The irrigation period starts at the specified crop start date when the crop factor becomes greater than zero. The area planted is based on the planting decisions for the associated season and a user-specified proportion (Equation

13) The fallow area is debited for the area of the crop planted (Equation

14). When the crop is planted the initial soil moisture is equal to the total soil moisture for the fallow (Equation

15).

where:

AP is the area to plant in the irrigation season (

m²)

A_F is the current fallow area (

m²)

A_c(c) is the area of crop c to be planted in the current season (

m²)

ncrop(s) is the number of crops in irrigation season s

P_c(c) is the proportion of area of crop c

M_t(c) is the initial soil moisture for crop c (m)

FUM_t-1 is the fallow upper soil moisture at the end of the previous time-step (m)

FLM_t-1 is the fallow lower soil moisture at the end of the previous time-step (m)

When the crop has finished growing, which is identified when the crop factor becomes zero, then the crop area (Equation

13) and moisture is transferred back to fallow (Equation

16 and Equation

17).

Dep_ratio is the proportion of Upper Soil in the soil profile under Fallow

where:

FUM_t is the fallow upper soil moisture at the end of the current time-step (m)

FLM_t is the fallow lower soil moisture at the end of the current time-step (m)

FUM_t-1 is the fallow upper soil moisture at the end of the previous time-step (m)

FLM_t-1 is the fallow lower soil moisture at the end of the previous time-step (m)

M_t-1(c) is crop c soil moisture at the end of the previous time-step (m)

A_c(c) is the area of crop c (

m²)

A_F is the fallow area (

m²)

The crop soil moisture is depleted by evapotranspiration and seepage and is replenished by rainfall and irrigation application. The objective of irrigation watering is to maintain the soil moisture at a target state. The target state may be fixed at half the soil moisture capacity, specified in a soil moisture target curve or for ponded crops (eg. rice) a minimum pond level can be specified. Optionally a crop tolerance (see Figure

1) can be specified as well as the target moisture state. In regulated systems, orders are placed when the soil moisture content is below the target moisture state less the crop tolerance, with the aim of bringing the soil moisture up to the target state plus crop tolerance. When meeting opportunistic requirements in regulated or unregulated systems, the aim is also to bring the soil moisture up to the target state plus crop tolerance.