xxiv Abstract This thesis, for the first time in Ireland, uses a framework that combines a land surface scheme (LSS) based on a surface energy budget theory, available environmental observations, land surface and atmospheric analyses, to understand essential mechanistic factors that determine grass growth response across the Irish landscape. A soil moisture model parameter (C soil) is identified as the key factor that distinguishes soil types and their ability to retain water for plant growth, plant response to exchange processes, and drives the response of LSS in drying soils. A Modification of this parameter indicates that the LSS can be transferred to other locations. In the context of understanding the links between land surface dynamic processes and the persistence of 2018 summer drought regionally, drying soils and high atmospheric anomalies result in a reduced evapotranspiration (ET) process. This is the situation over grasslands in the east and south east of the country where a wet ‘evaporative’ regime quickly shifts into a ‘transitional’ regime in which vegetation functioning and ET are controlled by soil water availability. Particularly, a threshold value of soil moisture content that suggests the onset of 2018 agricultural drought has been found across the regions. The importance of water use efficiency for monitoring grass growth at field level and for distinguishing zones of optimum productivity is further discussed in the thesis. Overall, the findings demonstrate the potential consequences of climate change on Irish grasslands and the need for policies that are tailored to reinforcing observation networks to complement theories and model outputs akin to on-farm adaptation and optimization of water availability and productivity.