Agricultural Demand Methodology

Unlike municipal demands which are typically fixed or follow strict daily patterns, agricultural water demand in IAMDD is highly dynamic. The platform does not assume a constant flow rate for irrigation; instead, it executes a continuous, daily soil-moisture mass balance for every agricultural polygon on the map. This technical reference outlines the primary calculations the engine uses to determine exactly when and how much water a specific field demands from the hydraulic network.

1. Reference Evapotranspiration (ETo)

The foundational metric for crop water demand is Reference Evapotranspiration (ETo), which represents the evaporative demand of the atmosphere.

• Data Source: IAMDD pulls daily localized weather data (minimum temperature, maximum temperature, precipitation, and solar radiation) for the exact geospatial coordinates of the drawn polygon.
• Calculation: The engine utilizes these variables to calculate a daily ETo value. Depending on the availability of regional climate variables, the system defaults to the standardized FAO-56 Penman-Monteith equation or the Hargreaves-Samani equation.

2. Dynamic Crop Coefficients (Kc)

Different crops consume water at different rates depending on their growth stage. The engine calculates the specific Crop Evapotranspiration (ETc) using the formula:

ETc = ETo × Kc Instead of using flat, static monthly averages, IAMDD tracks plant growth dynamically:

• Growing Degree Days (GDD): The engine tracks cumulative heat units (GDD) starting from the user-defined planting date.
• Canopy Curve: As GDD accumulates, the engine scales the Crop Coefficient (Kc) along a continuous growth curve—starting at $Kc_{ini}$ (bare soil), ramping up during the development stage to $Kc_{mid}$ (full canopy), and tapering off to $Kc_{end}$ during senescence and harvest.

3. Soil Moisture Mass Balance

To determine if irrigation is actually required to meet the calculated ETc, the engine maintains a running mass balance of the water stored in the crop's root zone. When an area is drawn, the engine queries the USDA SSURGO database to determine the field's specific soil texture. This provides three critical metrics: 1. Field Capacity (FC): The maximum amount of water the soil can hold against gravity. 2. Permanent Wilting Point (PWP): The soil moisture level at which plants can no longer extract water. 3. Available Water Capacity (AWC): The difference between FC and PWP (the water actually accessible to the crop). The daily balance is calculated as:

$SM_{t} = SM_{t-1} + Precipitation + Irrigation - ETc - Deep Percolation$ Any moisture that exceeds Field Capacity is assumed to be lost to Deep Percolation or surface runoff.

4. Irrigation Trigger Logic

IAMDD does not force irrigation to occur every day. Instead, it simulates realistic irrigation management using a Management Allowable Depletion (MAD) threshold.

• Trigger Event: When the calculated Soil Moisture ($SM_{t}$) drops below the MAD threshold (typically 50% of AWC), the engine triggers an "Irrigation Demand" event to refill the soil profile back to Field Capacity.
• Hydraulic Request: The required volume of water is converted into a flow rate (GPM) based on the size of the GIS polygon.
• Hardware Constraints: This requested flow rate is then passed to the Hydraulic Solver. The solver attempts to pull this water from the pipe network, constrained by two factors:
  1. Application Efficiency: The required flow is increased based on the efficiency of the assigned hardware (e.g., if a Center Pivot is 85% efficient, the system must pump more water to meet the net crop requirement).
  2. Maximum Flow Limits: The flow rate is capped by the physical maximum delivery capacity of the hardware or the maximum allowable flow through the delivery junction. If the pipe network (due to pressure losses, pump failures, or pipe sizing) cannot deliver the requested flow rate, the engine records an Irrigation Deficit, and the simulated soil moisture will continue to drop toward the Permanent Wilting Point.