Much variation but little change Rafael Rosa and Gerald Stanhill Institute of Soil, Water and Environmental research, Agricultural Research Organization, Bet Dagan

 Annual volume of rain falling in Israel is the most important although not the only factor determining the national water supply.  Nationwide approach to annual precipitations characteristics.

 Assess size and nature of inter-annual variation in national water supply.  Examine if any significant changes have occurred.

MAPS:  published by Palestine Mandate Meteorological Service.  Since 1948 provided by Israel Meteorological Service.  All the series are based on area within British Mandate borders.

 gravimetric method.  Since 1998 using AutoCad software.  Software’s model calibrated with previous analysis by comparison of two years’ results.  99% agreement.  Errors present in the whole analysis, starting from rain gage error at individual site.

mm rainfall

Statistical techniques used include:  Non parametric – needed to test significance of trends because of non independence of annual values in climate time series.  Spectral analysis – to test possible cycles, periodicity.  Standard parametrical methods – used to quantify trends.

 No significant trend observed during the whole period.  Changes in trend are also non significant.

 All values below line represent White Noise. Not significant.  Peaks at 2.7, 3.2 and 3.9 years may be associated with intensity of the Hadley Cell circulation system.  Peak at 13.5 years not previously reported.

 Non significant increase ( ± Km 3 per year)  Confirms previous results based on analysis of individual time series.

 No relation between annual rainfall volume and air temperature.

 Mean Volume = 7.97 Km 3  Median = 7.80 Km 3  Standard deviation = 2.03 Km 3

 Mean Volume = 7.97 Km 3  Median = 7.80 Km 3  Standard deviation = 2.03 Km 3

 28% of values outside ± 1 standard deviation limit.  3% of values outside ± 2 standard deviation limit.  1 value above ± 3 standard deviation limit (1991)

 There is a lack of very dry years or very dry spells.  Exceptional volume in 1991affects graph but not qualitative trend.

 There is a lack of very dry years or very dry spells.  Exceptional volume in 1991affects graph but not qualitative trend.

Length: not more than x years  More total runs above median than total runs below median – 15 above, 6 below  Longest run below median ( ).  More short runs above median. Wetter than medianDrier than median 2 years3 years4 years5 years6 years2 years3 years4 years5 years6 years

 Difference between observed and expected number of runs.  Short wet runs are more than expected. Wetter than medianDrier than median 2 years3 years4 years5 years6 years2 years3 years4 years5 years6 years Observed Expected

 Over the last 78 years there is no evidence for a long term trend in national water supply by rainfall.  The only non random feature of the rain series is the under representation of short dry runs.

 Considering a “run” as a sequence of values all higher (or lower) than the median.  Not considering length of each run.  45 runs detected.  Series homogeneous for 1%, 5% and 10% level of significance.

 For two cover types.