Manual de Plantas de Costa Rica | The physical environment | 85 
of Cafias and Bagaces), and the Gulf of Papagayo (Santa Elena Peninsula, Punta Gorda, 
etc.), all of which receive less than 1500 mm of annual rainfall (Herrera, 1985). 
In middle elevation, cloud-forest habitats, rainfall is supplemented by cloud water 
and mist (“occult precipitation”) and wind-driven (“horizontal’’) precipitation, neither 
of which is measured by standard rain gauges. Cloud immersion may approach 20— 
25% of the time at some sites in the Monteverde Reserve (Clark et al., 2000). Evidence 
suggests that wind-driven cloud water and mist may augment annual precipitation (as 
recorded using standard gauges) by as much as 20—150% or more, especially on wind- 
ward slopes. For example, at a site near Monteverde, during a year in which 3191 mm 
of rainfall was registered by a standard gauge, an additional 886 mm (=28%) of wind- 
driven cloud water was collected with a special cloud-water gauge (Clark et al., 2000). 
This site was located on a leeward slope. Costa Rican stations with frequent or heavy 
fogs or mists are characterized by a high degree of epiphytism, but it is unclear whether 
this reflects a specific response to cloud-water accretions or a more general response to 
the nearly year-round surplus of precipitation over water need (Holdridge et al., 1971). 
See Holdridge et al. (1971) and Clark et al. (2000) for more detailed considerations of 
this still poorly understood topic. 
Rainfall amount and seasonality are not the only hydrological factors that affect 
vegetation type. Of course, very dry conditions in a low, hot region will invariably re- 
sult in a desert flora. However, the same level of rainfall at a higher, cooler elevation, 
hence with a lower rate of evapotranspiration, may support lusher vegetation. This con- 
cept is a key to understanding the Holdridge life-zone system (see chapter on Vegeta- 
tion). The Monteverde Reserve does not receive notably high average annual precipi- 
tation (ca. 2500 mm at 1460 m elevation on the Pacific slope, adjusted for estimated 
cloud-water contribution); yet, it supports famously lush and rich vegetation, with an 
abundant and diverse epiphytic component, even relative to very wet, aseasonal low- 
land sites such as La Selva. The highest evapotranspiration rates in Costa Rica occur in 
the lowlands, especially in the northern Pacific regions. The average annual rates at 
Nicoya and Puntarenas exceed 1700 mm, peaking during March through May (Herrera, 
1985). Puerto Limon, on the Caribbean coast, averages just over 1500 mm per year, 
with a peak in May. High montane locales account for the lowest evapotranspiration 
rates, with Villa Mills averaging less than 600 mm per year (lowest from November 
through February). 
Relative humidity, closely linked to evapotranspiration and likewise related to tem- 
perature, precipitation, and windiness, has not been well documented in Costa Rica 
(Gomez P., 1986). Generally speaking, the highest values are found on the Caribbean 
slope, with high humidity year-round and an estimated annual average of 90%. During 
periods of cloud immersion, relative humidities of 95—100% are attained in cloud-forest 
habitats near the Continental Divide, as at Monteverde (Clark et al., 2000: 33, 34). At 
the other end of the spectrum, the average relative humidity during the dry season in the 
northern Pacific region is just 60% (Gomez P., 1986). 
