Modelling nocturnal heat dynamics and frost mitigation in Andean raised field systems

The raised fields system is an old technique of soil and water management dating back to prehispanic time. Very common in the Lake Titicaca region, it essentially consists of a series of earth platforms on which crops are grown, surrounded by water canals connected to inlet and outlet ditches. An important and widely recognised benefit of this system of management is its contribution to frost mitigation during the growing season. A physical process-based model is presented to explain the role played by the canals in the nocturnal heat dynamics and the cold mitigation process. Adapted from a two-layer transfer scheme (Shuttleworth–Wallace type) with a vegetation layer and a substrate layer representing the canals, the model shows that greater heat flux emanating from the canals and greater water condensation on the crop both contribute to the mitigation effect. Model outputs are compared with experimental data collected on a system of raised fields in the Lake Titicaca region. Crop temperature appears to be correctly estimated and more accurately than water temperature. When used in a predictive way, the model shows that wider canals or narrower platforms have a positive impact on the minimum crop temperature reached during the night. Increasing water depth also improves frost mitigation, but conversely, a deeper canal (with the same level of water) has a negative impact. Leaf area index (LAI) and crop height, both have a positive impact. Higher wind velocity or higher air relative humidity also enhances the frost mitigation effect.