Water balance and allocation model

Description

Water allocation is a decision about who gets water when it is scarce, and it is normally taken on the basis of records and negotiation. This model makes it computable. Built in RIBASIM, a river basin simulation package developed at Deltares since the 1980s, it links the water entering the basin at various locations to the users who take it out.

The Medina del Campo schematisation represents the three rivers that cross the municipality — the Duero, the Zapardiel and the Trabancos — and eighteen sub-basins. Each sub-basin has an inflow node where rainfall time series are introduced, a diverted flow link representing groundwater extractions, and an irrigation node at which the crop types and their demands are represented. The model is fed with irrigation demand by crop type, rainfall and discharge series, and groundwater extractions based on the permitted annual volumes; most of the data come from the Duero River Basin Authority and the regional government. Simulations run at half-monthly time steps, and the soil layer considered is the upper part, which allows the effect of different cropping patterns to be represented in detail.

The model assesses the current balance between available groundwater resources and irrigation demand, and the impact of the extraction wells on the sustainability of the aquifer. Its intended extension is the comparison of scenarios — different cropping strategies and different extraction policies — and the evaluation of how each affects infiltration and runoff.

Applications to decision making in the insurance sector

The model answers the question that determines whether a drought risk is manageable or structural: at what level of demand does the system fail, and what change in cropping or extraction moves that threshold? For a risk carrier with exposure to agricultural drought, that is the assessment underlying any view on whether a basin's risk is being reduced or transferred to the future.

Added value of this modelling approach

It joins the physical water balance to the agricultural economy of the basin, so a change in cropping is assessed both as an agronomic decision and as a change in water demand. That is the form in which the decision is actually taken.

What decision can be improved with this modelling approach

How much water can be allocated to each use without depleting the aquifer; which cropping pattern reduces demand while remaining viable for the farms; and how a change in extraction policy affects what reaches the rivers and the wetlands.