IWD: Irrigation water demand model
IWD is primarily designed as a pre-processor for the water resources model WRM:
it generates (daily) time series of supplementary irrigation water requirements
that can be used as the demand time series of an irrigation demand node.
The model describes the dyamic behavior of an irrigation district
or a similar spatial and administrative unit of irrigated agricultural land.
The unit the model describes is assumed to have homogeneous soil parameters,
a shared irrigation technology, but can have several crops with their respective (area) shares.
The model includes an optional (local) buffer reservoir associated with the
irrigation district. This can also represent storage volumes in the primary
irrigation canals. Water is extracted from the river system to fill the buffer reservoir,
starting with a user defined lead time before the first planting date
Water for irrigation is then taken from the buffer reservoir rather than the river directly.
The buffer reservoir is replenished as needed (subject to an abstraction capacity constraint),
and drawn down to zero storage at the end of the irrigation period.
A zero sized reservoir will lead to the irrgation requirements
being directly taken from the extraction point.
Within the WaterWare network architecture, a detailed distributed representation
of any irrigation district with numerous fields and primary and seondary irrigation canals,
buffer reservoirs, and several alternative sources of water can be built by a
coupled network of several parallel or cascading irrigation nodes.
A scenario for the irrigation model is an object like other model scenarios;
it is primarily described by a set of META data:
Specific attributes of a scenario include:
- Scenario name and description;
- Author, creation and last modification date;
- Location (geographical domain assisting the choice of input data sets).
- Object link (to an irrigation type river basin object);
- Irrigation technology
- Planting time shift; the model uses th planting dates of the crops defined by the scenario;
however, the water demand (within the calendar year) can be delayed by a sgift, expressed in days.
This shift, however, applies to ALL crops selected equally.
To modif the planting dates of individual crops, the corresponding parameter (planting date)
can be edited with the individual crops on the crop selection page.
- Initial soil moisture (in % of field capacity);
- Total area in ha;
- Predominant soil (selected from a list of pre-defined
soil types, described
with a combination of soil texture and soil organic content))
- A buffer reservoir: as part of the scenario a hydraulic buffer (reservoir)
can be defined, representing both a specific storage reservoir, and, depending on
the relative size, the main irrigation channel network's storage volume.
The buffer reservoir enables local storage against varying river flow;
the use of more constant (and cost efficient)
pumping strategies, or to balance constraints in the primary sources (diversion,
gravity or pumped flow from a storage reservoir, groundwater wells)
to satisfy short-term peak demands.
The related parameters include:
- The reservoir storage capacity
- The reservoir surface area (we assume a simple rectangular cross-section);
- A time period for pre-filling the reservoir, defined by the number of days
that filling can start BEFORE the crop water demand starts.
- Seepage losses for the reservoirs (in mm/m*day)
- A maximum diversion flow and/or pumping rate (combined) available
to fill the reservoir from the extraction point.
Please note that the maximum supply and application rate
to the fields from the buffer reservoir can be constrained by
the capacity of the irrigation technology selected !
- Correction factor for effective precipitation (multiplier: 0 < MULT < 1.0);
- Degree-day coefficient for evaporation (from soil moisture);
this is a minimum parameter estimate that erquires only daily average temperture;
- Multipliers for temperature and precipitation data to
represent different meteorological scenarios such as wet and dry years.
These comprise two groups:
- crop and cultivation specific dynamic data;
- hydro-meteorological (daily) data.
Time series of crop specific data are specified for dates every of 1/10 of the growing period
and interpolated to daily values.
Dynamic inputs include daily time series of hydrometeorological variables that
can be selected from the model TS data basei, and/or edited there by the user,
including general multipliers in the scenario. The hydrometeorological time series can
be also calculated with the (optional) prognostic, non-hydrostatic meteorological models WRF or MM5,
based on operational forecasts (GFS) or re-analyzed historcal data sets (NCEP-DOE, ECMWF ERA-15).
- CWR, crop water requirement: IWD uses a concept of dynamic plant physiological water
demand directly (assuming otherwise optimal conditions, e.g.,
in terms of temperature or nutrients) and estimating soil moisture evaporation from air temperature
rather than a multiplier on potential evpotranspiration);
- MAD (management allowed depletion) in % of field capacity, allowable values would range from 0
(soil moisture is always maintained at field capacity to 90 (soil moisture can drop by 90% to 10% of
field capacity or PWP (Permanent Wilting Point).
for paddy cultivation, this describes the allowable drop
in the water level below the target level:
- WTL (water target level),
specified only for paddy cultivation. Water above that target level will be drained, subject
to the capcity of thye drainage system.
- FAO Kc values (for CROPWAT fans);
- Shading that describes the relative reduction of evaporation due to
shading of the soil by the developing plant canopya in %.
Distributed data include:
- Air temperature (from which soil evaporation is estimated),
the user can supply a time series (from the model TS data base,
- Precipitation; time series of daily precipitation values selected by the user from the model TS data base.
- Crops: they are part of a scenario, but therea is also an external,
shared data base of crops from which the scenario version can
select and inherit crop properties. Crops are similar to any RBO without the georeference,
but region or location specific data such as planting time and growing period.
- Crop name: their specific water demand data
are then loaded automatically from a simple crop data base.
- Relative share of the total area;
- Planting date (growing period is taken from the data base - this is curently fixed with the crop,
but should be adapted to the local climate, temperature and sunshine hours).
- Irrigation technologies and their associated
data on efficiencies or losses and target and maximal application rates
are taken automatically from the data base.
The model estimates supplementary irrigation water requirements to maintain
optimal conditions for plant growth: for that it maintains
a soil moisture level between field capacity and MAD: Management Allowed Depletion,
expressed as a percentage of field capacity (e.g., 50%).
If the lower MAD threshold is being reached, supplementary irrigation water is added;
How much depends on irrigation technology: for each technology, a target (expressed as %
of MAD (where 100% means up to field capacity, and 0 means up to MAD level, i.e., it describes an irrigation
technology specific excess of water (above MAD) that can be supplied, to smooth oscillations of soil moisture
or water levels in paddies) and a maximum technology specific daily application rate in mm can be specified.
The defaults are 0 % (only apply up to MAD) and unlimited (whatever amount of
water is required to reach the target soil water content.)
The MAD level for a crop is either contant, or can optionally be specified as a time series of 10 values,
each representing 1/10 of the crowing period.
For Paddy Cultiviation irrigation water is supplied to maintain the pre-defined
water (target) level in the paddy; excess water (e.g., from rain or when the target is lowered)
is drained, inflow and outflow are subject to the site specific maximum flow constraints.
Excess water from rainfall above field capacity (or maximum depth for paddy cultivation)
is lost to percolation to the groundwater, or in the case of rice paddies will overlow.
The (default) buffer between field capcity and MAD is the soil moisture
reservoir that rainfall can replenish.
For Paddy Cultiviation excess water (above the current target level) gets drained
contributing to the return flow from the site.
The model estimates a weighted average loss of irrigation water applied
based on irrigation technology.
For every time step, the model generates a simplified water budget by:
- Estimating demand:
- Sum of all individual crop water demands ;
- Evaporation from the soil system as a function of air temperature.
Please note that soil evaporation is
optionally modified by the plant canopy (shading), and by soil moisture.
Since this can drop below PWP (permanent Wilting Point) outside the growing season
when no irigation requirements are computed and simulated, evaporation
DECREASES linearily with soil moisture, having its maximum at field capacity, and
decreasing to 0.0 at PWP, set at 10% of field capacity.
- Estimate of effective rainfall (corrects for interception
and evaporation on the canopy, as well as any runoff);
excess rain beyond field capacity is being lost to percolation;
the relevant soil layer is determined by a plant specific root zone thickness);
- Estimate the difference: supplementary daily irrigation water requirement,
corrected by the technology dependent loss type.
With minor modifictaions, the model is also applicable to wet rice paddy cultivation.
The differences in concepts and parameters are:
For the land preparation, an additional 150-250 mm have to be added.
They are accounted for at the beginning of the growing season by
specifying the corresponding sets of target water levels.
For rice paddy cultivation, only flooding (gravity feed or pumped)
is applicable as an irrigation technology.
As an additional parameter, a seepage rate (includes both vertical and lateral losses)
from the paddy to the underlying soil/aquifer system is considered:
this is using a rate constant that describes seepage as a function of water level
on the paddy field.
Total water requirements and specific water use (in mm)
for rice production under different ecologies can be roughly
estimated on average between about 500 to more than 1,500 mm;
Out of this evapotranspiration ranges from 500-1,000 mm,
which is the water actually consumed by the plant/soil system.
|standard crop or plantation
||rice paddy cultivation
|field capacity in mm/m||maximum depth in mm (bund height)
|MAD as % of field capacity||MAD as % of maximum depth
|water loss due to seepage above field capacity||drainage above maximum depth
|all irrigation technologies applicable||irrigation technology restricted to flooding only
| ||initial field preparation requirements
| ||percolation and lateral seepage, as a function of depth
| rootzone depth ||
||demand in (mm), description
|rainfed upland rice||550
|rainfed lowland rice||1,000
||evapotranspiration + impounded rainwater||2.50||4.0
|irrigated upland rice||1,000
||evapotranspiration + supplementary irrigation||2.50||4.0
|irrigated lowland/deepwater rice||1,650
||evapotranspiration and full irrigation||4.50||3.7
Data Tables used by IWDM
Crops (external data base):
an integrated data base currently (R6.1) holds data on 146 crop the user of the irrigation model can select from;
the crop data base if open and can be extended by the user as needed for a specific application.
- Identification and META data with
- Common name,
- Scientific name,
- Short description (including source and reference),
- Hypertext link including imagery),
- Author, creation and modifiction data.
- Location for which the data are applicable;
- Growing period in days (location specific);
- Start (planting date or start of growing period, e.g., for fruit trees, location specific.
Multi-annual plants such as fruit tress can use a growing period of 365 days,
with the corresponding seasonal pattern of water demand);
- Root zone thickness;
- Groundwater level: LOW - MEDIUM - HIGH (affects soil moisture);
- Kc (constant, optionally overaloaded with a time series)
- OPTIONAL: min/average/max yield, world market price (and reference year).
- Time series
these are described as generic patterns of 10 values for 10% of the growing period each;
together with the Growing period, a time series for the N days of the growing period
can be generated dynamically.
- Physiological crop water requirement (CWR, in mm/day)
- OPTIONAL: Kc values (default: constant Kc);
- OPTIONAL: soil moisture evaporation reduction factor for canopy development
(default: 1.0, no reduction).
Soils: Name, field capacity.
A set of 15 soil types (defined by the combination of structure and organis content)
is used, consisting of 5 structural classes together with
3 organic content levels.
A separate pop-up page provides and editor for the irrigation technology