The climatic factor that most affects yields in rainfed crops is precipitation, mainly as rain during the growing season.
The water availability of the crop results from the soil water holding capacity, precipitation and evapotranspiration (ETP). ETP consist of water losses from a field through evaporation of moisture from the soil surface and from transpiration of water from the leaf surface of the potato plants. Transpired water first is taken up by the roots of the plant and evaporates through the leaves. Evapotranspiration increases with lower relative humidity of the air, with higher solar radiation, higher temperatures and a higher wind speed. The Penman-Monteith equation makes use of these variables to calculate the ETP when measurements of pan evaporation on the spot do not take place.
When ETP exceeds precipitation and plant-available ground water nears exhaustion, crop growth is limited by the available amount of water. Soil characteristics that determine the availability for the potato crop are its waterholding capacity and its rooting depth mainly. The water-holding capacity is the amount of water expressed in mm that the soil contains per meter soil depth. It is mainly determined by the granular composition of the soil. Very light sandy soils and very heavy clay soils withhold little water after rainfall or irrigation whereas intermediate soil types contain optimal amounts of water. The total amount of water available to the crop after water is supplied, is not only determined by water-holding capacity. It also depends on the rooting depth of the soil that varies anywhere between 20 cm up to 100 cm from the top of the soil. Other soil factors such as acidity (pH), salinity, sodicity, stoniness and slope also limit production when they reach unfavorable levels.
A rain gauge measures the amount of rain fallen in a set period, usually a day. Registration takes place as mm (here and there in inches) or liters per m2. Radar observations yield information about spatial shower patterns and acoustic observations about drop size distribution.
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