Dependable rainfall is that rainfall which can be

Dependable rainfall is that rainfall which can be expected a set number of years out of a total number of years. For agricultural production expendable rainfall is normally understood as the rainfall which can be expected in 3 out of each 4 years or in 4 out of each 5 years 10/22/2021 3

DEPENDABLE RAINFALL : DRY , NORMAL AND WET YEARS 1 Upon the determination of rainfall frequencies and / or probabilities , the question arises how to use this information. Questions which may be posed are following : 10/22/2021 4

)��� ( 1. What is the rainfall received four out of each five years. This information is of importance to ascertain if rainfall is sufficient to support rainfed agriculture or if supplementary irrigation is needed 2. What is the average or mean rainfall i. e. the rainfall to be expected each other year. 3. What is the “ maximum ” rainfall received once each five years. 10/22/2021 5

To answer these questions the concepts of “wet” , “normal” , and “dry” years have been introduced. -- A “dry” year denotes a year with an annual rainfall , which will be exceeded 4 out of 5 years – probability of exceedance of 80% -- A “normal” year is a year with an annual rainfall , which will be exceeded 1 out of 2 years i. e. each other year. The probability of exceedance is equal to 50% and the rainfall in the “normal” year corresponds to the average or mean monthly and annual rainfall. -- A “wet” year denotes a year with an annual rainfall , which will be exceeded 1 out of 5 years – probability of exceedance 10/22/2021 Exam Analysis 6

Effective rainfall is defined as the fraction of rainfall, that is effectively intercepted by the vegetation or stored in the root zone and used by the plant soil system for evapotranspiration 10/22/2021 8

Effective Rainfall • For agricultural production, effective rainfall refers to that portion of rainfall that can effectively be used by plants. This is to say that not all rain is available to the crops as some is lost through Runoff (RO) and Deep Percolation (DP). • How much water actually infiltrates the soil depends on soil type, slope, crop canopy, storm intensity and the initial soils water content. The most accurate method to determine effective rainfall is through field observation. Rainfall is highly effective when little or no RO takes place. Small rainfall amounts are not very effective as these small quantities of water are quickly lost to evaporation. • As input of monthly rainfall, the average, dependable or actual rain fall data can be given. Care should be taken in select ing ap propria te val ues for the dependable rainfall, based on separately carried out stat istical ana lyses of long term rainfall records. 10/22/2021 9

Through the Rain Options, CROPWAT 8. 0 offers the possibility to use several methods to calculate the effective rainfall: 1. Fixed percentage of rainfall 2. Dependable Rain 3. Empirical formula 4. USDA Soil Conservation Service Method 10/22/2021 10

����� 1. ���� Fixed percentage Effective rainfall ������� for a = 0. 7 to 0. 9 Peff ������� Ptot �������� : ��. 10/22/2021 11

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The FAO CROPWAT program 10/22/2021 19