The impact of irrigation on local weather

Water availability from irrigation practices has enabled agricultural regions to expand into areas previously unable to support food production and plays a critical role in regional food security. To what extent has this expansion in irrigated regions had a feedback on the local climate? Climate warming will see drought events occurring with increasing frequency, intensity, and duration which places a greater reliance on irrigation practices for food security. The resilience of irrigated agriculture to drought is essential. Can irrigation practices be changed to minimise this impact and at the same time minimise water loss through evaporation? This project will exploit observations from the LIAISE (Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment) campaign to investigate the role of irrigation in modulating the regional boundary layer evolution. Irrigation strongly modulates surface exchange leading to large spatial variations in atmospheric temperature and moisture.

This project will explore the resulting modification of regional winds within and downwind of irrigated areas leading to the generation of secondary circulations and the modulation of heat and moisture advection. Current Unified Model (UM) configurations fail to represent irrigation globally and so incur systematic biases linked to dry soils over irrigated areas. These biases are not just localised at the surface but extend through the atmospheric profiles of temperature and humidity and impact on regional scale circulations, which has implications for regional rainfall. LIAISE observations will be used as an evaluation testbed to understand the impacts at a regional scale and improve process understanding and model development in a way that can be applied globally.

In this project the student will;

• run and evaluate the UM at km & sub-km scale with the UM irrigation scheme to address the forecast biases and understand the importance of local changes in fluxes of heat and moisture versus changes in advection of heat and moisture through induced local circulations.
• examine the pre-convective boundary layer in the km & sub-km scale simulations to explore how this is affected by surface heterogeneity, both naturally occurring, and irrigation induced heterogeneity.
• evaluate the ability of the UM at sub-km scale to capture local scale turbulent processes, boundary layer characteristics and growth at ~100m resolution.
• use km & sub-km simulations to study how boundary layer structures interact with mesoscale circulations induced by land-surface heterogeneity.
• conduct further tests to understand the sensitivity of the results to the way the irrigation is applied, in particular the timing and amount of irrigation. These tests will be used to evaluate what level of detail is needed in an operational irrigation scheme, but also how irrigation might be modified to minimise water loss to evaporation and limit impacts on the local weather.
• investigate the implications for the parametrization of surface exchange and boundary layer turbulence, particularly in coarser resolution models where the patches of irrigated land and the induced circulations may be poor resolved or even completely unresolved.

LIAISE was a major international field campaign in north-eastern Spain which took place between June 2021 and July 2022. The study region covers approximately a 30×30 km square area and had two clearly defined zones; one that is irrigated and one that is naturally rainfed, but arid during the summer months. A network of eddy-covariance flux sites were deployed across the region during June-September 2021; 5 sites located in the irrigated zone and 3 sites located in the rainfed zone. Each site was representative of a different dominant crop grown in the region or the natural vegetation to provide a measure of the surface heterogeneity. Two supersites were deployed (one rainfed, one irrigated) with a 50 m flux tower and comprehensive boundary layer remote sensing instrumentation. A two-week Intensive Observing Period was carried out during July 2021 with radiosonde sonde launches and airborne observations. At the Met Office supersite, the 50-m flux tower observations were extended for a 12-month period throughout 2021-2022 to obtain observations of the soil moisture dry-down period in spring 2022, important to understand the processes giving rise to surface heterogeneity in the region. A stable boundary layer experiment was also carried out during spring 2022 with 7 IOPs achieved including intensive radiosonde observations.

The student will join a wider group at Leeds and the Met Office working on land-atmosphere interaction including work under the WCSSP programme on irrigation impacts in India and the NERC-Met Office Turbulent Processes programme which includes work on surface heterogeneity impacts on the boundary layer and their representation in km and sub-km models.