Much variation but little change Rafael Rosa and Gerald Stanhill Institute of Soil, Water and...

Much variation but little change

Rafael Rosa and Gerald Stanhill

Institute of Soil, Water and Environmental research, Agricultural Research Organization, Bet Dagan 50 250

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: 1930-1947 published by Palestine Mandate

Meteorological Service. Since 1948 provided by Israel

Meteorological Service. All the series are based on area within

British Mandate borders.

1931-1998 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

mm rainfall

mm rainfall

mm rainfall

mm rainfall

mm rainfall

mm rainfall

mm rainfall

mm rainfall

mm rainfall

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 (0.0002 ± 0.0103 Km3 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 Km3 Median = 7.80 Km3 Standard deviation = 2.03 Km3

Mean Volume = 7.97 Km3 Median = 7.80 Km3 Standard deviation = 2.03 Km3

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 (1957-1962). More short runs above median.

Wetter than median Drier than median2 years 3 years 4 years 5 years 6 years 2 years 3 years 4 years 5 years 6 years

13 1 1 0 0 1 3 1 0 1

Difference between observed and expected number of runs.

Short wet runs are more than expected.

Wetter than median Drier than median2 years 3 years 4 years 5 years 6 years 2 years 3 years 4 years 5 years 6 years

Observed Observed13 1 1 0 0 1 3 1 0 1

Expected Expected10 5 2.5 1.25 0.65 10 5 2.5 1.25 0.65

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.