Modelling Crops Water Foot Print

According to FAO, worldwide agriculture accounts for over 70% of blue water consumption. The anticipated future increase in global population, from 6.1 billion people in 2000 to 8.9 billion by 2050, entails the increase of agricultural production. It is estimated that by 2030 50% more food has to be produced, and twice the current amount of food by 2050. However, this increase in food production should be carried out with the least amount of water needed, mainly due to increases in urban and industrial water consumption and possible consequences of climate change. It will be necessary to increase the water needed for food production from the current 7.000 km³ to 9.000-11.000 km³ by 2050.

Spain is no exception, water consumption is strongly geared towards the agricultural sector, and the rate of exploitation of renewable resources exceeds 30%. In the past 20 years, the irrigated area has raised to 20%, causing a large increase in water demand. This makes Spain the country with the largest irrigated area in Europe, with nearly one third of the total European irrigated area. Nevertheless, Spain remains a net importer of virtual water, with more than 25,000 million m³ per year, mainly associated with the import of cereals and industrial crops such as soybeans or cotton. But more and more criticism arises against the agriculture sector remaining the centre stage. The old paradigm "more crops and jobs per drop" is shifting towards “more cash and nature per drop". Determining the current and future water demands will help to implement sustainable policies for water resources management.

Water use at a national level has traditionally been measured by indicators such as water withdrawal, which only considers the total freshwater used by a country in its production system. The use of indicators such as the Water Footprint (WF) allows analysing not only the impacts generated at the national level, but all those associated with the consumption of goods produced abroad. This multidimensional indicator distinguishes also between blue water (surface and groundwater) and green water (water from rain accumulated in the soil).It is furthermore possible to quantify the impact of pollution by calculating the gray water, which is defined as the total freshwater required to assimilate the load of pollution.

The Virtual Water (VW) concept was defined by professor Tony Allan in the beginning of the 90´s; since then, notable advances in the development of the Water Footprint concept have been achieved. The first major quantification of water flows associated with trade of commodities was made by Chapagain and Hoekstra. They established the VW flows of several crops and derived products. Nowadays, this methodology is standard, thanks to the efforts of the researchers of the Water Footprint Network. Methodological advances include the use of complex geographical models to estimate the water use of crops (CWU)(Mekonnen and Hoekstra, 2011; 2012). These models are based on water balances equations. They allow for the estimation of the amount of water embedded in crops in a certain area and at a given time.

Water balance models can be developed at different time and spatial scales, thus they vary in complexity and input data. There are several models to calculate crop water requirements on a global scale. Some of the most recent ones have been implemented with a resolution of 5 minutes and a total of 26 crop classes (both for rainfed and irrigated conditions). Some other models have been was applied to 126 crops, including calculations of gray water. But, the certainty of these models is influenced by the input data: location of crops and planting dates, weather variables, soil properties, etc. The total available water capacity of the soil (TAWC) plays a critical role in determining the overall water balance because it acts as a water reservoir.

To sort out some of such drawbacks researchers (A. de Miguel, M. Kallache and E. García-Calvo) have developed a spatial crop water use model (CWUModel), which computes the Water Footprint of agriculture in the Duero river basin. This model enables daily water balances in a geospatial context, distinguishing between green water and blue water.

The work can be accessed at our Cuadernos de Geomática Aplicada section.