#Seasonal precipitation totals zip#

The design rainfall is determined by means of a statistical probability analysis. If, for example, the design rainfall with a 67 percent probability of exceedance is selected, this means that on average this value will be reached or exceeded in two years out of three and therefore the crop water requirements would also be met in two years out of three. The design rainfall is usually assigned to a certain probability of occurrence or exceedance. If the actual rainfall in the cropping season is below the design rainfall, there will be moisture stress in the plants if the actual rainfall exceeds the design rainfall, there will be surplus runoff which may result in a damage to the structures. At more arid locations it is not uncommon to experience several consecutive years with no rainfall.įor a water harvesting planner, the most difficult task is therefore to select the appropriate 'design' rainfall according to which the ratio of catchment to cultivated area will be determined (see Chapter 4).ĭesign rainfall is defined as the total amount of rain during the cropping season at which or above which the catchment area will provide sufficient runoff to satisfy the crop water requirements. With mean annual rainfalls of 200-300 mm the rainfall in 19 years out of 20 typically ranges from 40 to 200 percent of the mean and for 100 mm/year, 30 to 350 percent of the mean. In arid and semi-arid climates the ratio of maximum to minimum annual amounts is much greater and the annual rainfall distribution becomes increasingly skewed with increasing aridity. In temperate climates, the standard deviation of annual rainfall is about 10-20 percent and in 13 years out of 20, annual amounts are between 75 and 125 percent of the mean. Water harvesting planning and management in arid and semi-arid zones present difficulties which are due less to the limited amount of rainfall than to the inherent degree of variability associated with it. This relationship appears to be independent of the long-term average rainfall at a particular location. Studies carried out in Saudi Arabia (Raikes and Partners 1971) suggest that, on average, around 50 percent of all rain occurs at intensities in excess of 20 mm/hour and 20-30 percent occurs at intensities in excess of 40 mm/hour. Analysis of short-term rainfall data suggests that there is a reasonably stable relationship governing the intensity characteristics of this type of rainfall. The statistical characteristics of high-intensity, short-duration, convective rainfall are essentially independent of locations within a region and are similar in many parts of the world. Rainfall intensity is defined as the ratio of the total amount of rain (rainfall depth) falling during a given period to the duration of the period It is expressed in depth units per unit time, usually as mm per hour (mm/h).

When most precipitation occurs during winter, as in Jordan and in the Negev, relatively low-intensity rainfall may represent the greater part of annual rainfall. However, low intensity frontal-type rains are also experienced, usually in the winter season. Precipitation in arid and semi-arid zones results largely from convective cloud mechanisms producing storms typically of short duration, relatively high intensity and limited areal extent. There are, in addition, other important factors which influence the runoff generating process.

Runoff is generated by rainstorms and its occurrence and quantity are dependent on the characteristics of the rainfall event, i.e.

#Seasonal precipitation totals zip#

Weekly rainfall totals by zip code.As defined in Chapter 1, water harvesting is the collection of runoff for productive use.