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.

Irrigation Scenarios

A scenario for the irrigation model is an object like other model scenarios; it is primarily described by a set of META data:

1. Scenario name and description;
2. Author, creation and last modification date;
3. 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. They include:

1. 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);
2. 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:
3. 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.
4. FAO Kc values (for CROPWAT fans);
5. Shading that describes the relative reduction of evaporation due to shading of the soil by the developing plant canopya in %.

1. Air temperature (from which soil evaporation is estimated), the user can supply a time series (from the model TS data base,
2. Precipitation; time series of daily precipitation values selected by the user from the model TS data base.

1. 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).
2. 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.

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,
  • Type,
  • 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.

  Irrigation Technology:

A separate pop-up page provides and editor for the irrigation technology parameters.