Defining a Dry Climate

It seems intuitively obvious, right?  Dry climate means little precipitation … but how much is “little?  There are degrees of precipitation that have been incorporated into a climate classification system, Koppen-Geiger, that is based on precipitation, temperature, humidity, and more.

Koppen_World_Map_(medium)

 

But there is one key aspect to the classification of “dry climate” that isn’t so cut-and-dried as merely direct measurement parameters. When examining climate variables, a clear demarcation can be discerned by analyzing the direct measurements to determine the relationship between precipitation and evaporation.  A “dry climate” can be defined to be one in which evaporation (including transpiration) exceeds precipitation over the course of a year, i.e. evapo/transpiration > precipitation.

Understanding Dry Climate Constraints

When we put on our Permaculture hat and begin to evaluate any given site in the observation phase of design, one of our very first concerns is water:  it’s sources, paths, and use.  Given a climate where evaporation exceeds precipitation, we are immediately struck with the conundrum of a net water loss within the baseline system.  This means that year-on-year more water evaporates or is transpired away by plants than rains from the sky;  that digging 5 or 10 feet down isn’t going to produce moist dirt, only dust;  that the land is parched and the native plants survive long spells without moisture between infrequent or paltry precipitation events.

Dry climates are classified into either BW (desert/arid) or BS (steppe/semi-arid) and may be characterized by the occurrence of tree species only within niche or micro-climates throughout the landscape.  Sunshine, lack of humidity, and wind all serve to quickly extract moisture from the ground and from un-adapted plants in these environments.

Reversing the Inequality

The relationship “evapo/transpiration > precipitation” allows no avenue to alter the precipitation  parameter … without drastic (and un-Permaculture-like) tactics like cloud seeding or HAARP.  This leaves us with evaporation and transpiration.

Transpiration through the stomata is the engine that drives the water, mineral, and nutrient flows from roots to shoots and evaporatively cools the plant.  The air environment around the plants leaves (humidity, temperature, wind and incident sunlight) as well as soil water supply and soil temperature can all affect stomata opening, and thus transpiration.  So, while we can see that transpiration is a necessary “cost” of plant health, there are some important dry climate parameters we can mitigate to minimize the plant’s evaporation of water through the leaves;  soil water supply, soil temperature, air humidity and temperature, wind, and sunlight are all partially within our control.

Strangely enough, these are also the parameters we need to affect to minimize evaporation from the soil!  Essentially, our goal in dry climates is to shield our plants from the wind, create shade, reduce excessive temperatures, and hold humidity in the air and moisture in the soil.

 

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