Rainfall is higher on the leeward (western) side of the island, especially on the western slopes of Centre Hills (Fig. 4). There is also a contrast in the relationship between elevation and rainfall in the east and west of the island (Fig. 5). The available rain gauge

data suggest that rainfall is ∼80% greater over the eastern peaks than on the coast; in the west it is >100% greater on the peaks. A paucity of instrumentation within the densely vegetated high elevation regions restricts the accuracy of this estimate. The spatial variation in precipitation is reflected in climax vegetation; the leeward (western) and elevated areas that are unaffected by the volcanic activity CHIR 99021 are covered in dense, tropical forest, while scrub, grass and cacti dominate the dry, windward (eastern) and northern slopes and coast. Groundwater recharge is a critical control on any subsurface hydrological system. In tropical islands such as Montserrat, high temperatures and dense vegetation can combine to produce high evapotranspiration rates, significantly reducing effective recharge. No evaporation pan measurements exist on Montserrat. In the absence of direct measurements, calculation of the potential evapotranspiration (PET) is necessary. The Thornthwaite method ( Thornthwaite, 1948) is one of the most commonly used of several empirical methods or used to estimate PET (see

Schwartz and Zhang, 2003). Akt inhibitor The method uses average monthly temperature to calculate an estimate for monthly PET. equation(1) PET=1.6210TaiIawhere PET is potential evapotranspiration in cm/month, Tai is the mean air temperature in °C for month i. I is the annual heat index given by: equation(2) I=∑i=112Tai51.5from which the constant a is derived: equation(3) a=0.492+0.0179I−0.0000771I2+0.000000675I3a=0.492+0.0179I−0.0000771I2+0.000000675I3 Thornthwaite estimates for PET on Montserrat vary between 100 and 150 mm/month, yielding a total 1500 mm/year ( Fig. 2). Thus PET is close to, and sometimes greater than, the average annual rainfall in some locations.

Only when soil water is not limited can actual evapotranspiration (AET) be assumed to equal PET. We use distributed recharge model P-type ATPase code ZOODRM (Hughes et al., 2008 and Mansour et al., 2011), to estimate spatially and temporally distributed AET from Thornthwaite PET calculations, by incorporating distributed, daily precipitation data and vegetation type information. We define four vegetation types based on land use maps from the Government of Montserrat: bare soil, grass-dominated (often anthropogenic), tree-dominated and fresh volcanic deposits ( Fig. 6). ZOODRM uses a soil moisture deficit (SMD) calculation to relate AET to the PET estimates in Fig. 2 and derive distributed recharge. Two major, depth related parameters are assigned to each vegetation type; the root constant (C) and wilting point (D) ( Table 1).