At a Glance
Human water management practices have a noticeable impact on the hydrological cycle. These include diverting water for irrigation, abstraction of groundwater, and construction of reservoirs. Hydrologic extremes, in particular, are heavily affected by water management practices, due to the existing stress on the system during droughts and floods. To prepare adaptation plans for hydrological extremes in the future, it is essential to account for water management and other human influences in state-of-the-art climate models.
This project focuses on the implementation of irrigation practices and reservoirs in the LM3 model, which is the land component of GFDL’s Earth System Model. Irrigation accounts for 70% of global freshwater abstractions, while reservoirs significantly change the hydrological regime of major rivers. It is therefore crucial to include these processes in the current model to improve our ability to accurately simulate the hydrological cycle.
Irrigation is estimated as the difference between the amount of water that plants need for optimal growth and the actual available water in the soil. This method was used to dynamically estimate irrigation demand for croplands across the continental US (Figure 1.7.1). The modeled estimates of total irrigation demand show a similar spatial pattern to reported irrigation water withdrawals, although there is a slight underestimation in some regions. Reservoirs in the model change the flow towards the rivers, where the reservoir release is dependent on the downstream irrigation demand and minimum flow requirements. Most water is released in summer, while in winter a net increase in reservoir storage is observed.
Including these irrigation processes led to changes in simulated vegetation characteristics and hydrological variables, such as increased vegetation growth over the croplands (Figure 1.7.2), and increased transpiration (i.e., evaporation by plants) and runoff. This led to a decrease in canopy temperature, with some locations showing a decrease of more than 8°C.
Apart from causing these changes, the water management practices also had a significant influence on drought across the US. The simulations indicated that irrigation aggravated drought conditions in rivers, because water was taken from the water ways to stimulate plant evaporation. In contrast, the inclusion of reservoirs increased the river outflow in large parts of the US. However, the effect of reservoirs is very much dependent on the operation rules of each specific reservoir. It is assumed that these rules are aimed at preventing and alleviating drought.
This new modeling framework provides a unique opportunity to study impacts of human water management at high resolutions, aiding a better understanding of anthropogenic impacts on vegetation growth, the hydrological cycle, and drought. In the future, the modeling framework will be elaborated with improved methods to estimate water management practices. Finally, the framework will be used for simulations at the global scale to be able to investigate human impacts across different climates.