041019 Arun at Alfoldean 2001
Measuring authority: EA Grid reference: 51 (TQ) 117 331 Catchment area (sq km): 139.0
First year: 1970 Level stn. (m OD): 21.40 Max alt. (m OD): 294
Daily mean gauged discharges (cubic metres per second)
DAY JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
1 12.790 2.754 4.686 1.988 3.833 0.338 0.237 0.174 0.306 1.921 0.802 6.484
2 11.160 4.869 2.743 1.974 2.973 0.334 0.237 0.397 0.269 6.126 0.734 3.034
3 8.594 8.264 1.878 3.720 1.885 0.353 0.251 0.385 0.312 1.558 0.693 2.613
4 14.350 13.830 1.577 5.391 1.355 0.326 0.238 1.080 0.404 0.724 0.672 3.185
5 12.680 13.360 1.334 5.554 1.089 0.314 0.264 0.738 0.301 0.553 0.619 6.386
6 5.700 11.560 4.263 3.778 0.926 0.318 0.239 0.734 0.316 0.986 0.589 3.023
7 3.812 8.580 4.929 2.713 0.848 0.322 0.631 0.506 0.252 10.540 0.616 1.827
8 2.821 13.820 7.821 6.470 0.787 0.318 0.560 0.898 0.243 14.360 0.889 1.372
9 2.486 6.435 4.199 9.080 1.103 0.326 0.354 2.167 0.225 5.409 0.697 1.189
10 2.283 5.090 3.065 3.762 1.912 0.320 0.322 0.995 0.217 2.002 0.582 0.994
11 2.005 5.513 4.788 2.655 1.334 0.301 0.309 0.450 0.233 1.174 0.557 0.801
12 1.744 11.020 7.344 1.872 0.890 0.294 0.339 0.335 0.269 0.997 0.598 0.854
13 1.512 6.961 6.371 1.650 0.758 0.288 0.284 0.306 0.318 0.829 0.576 0.817
14 1.394 3.979 3.408 1.664 0.699 0.297 0.379 0.279 0.253 0.803 0.501 0.743
15 1.345 3.077 3.027 2.514 0.751 0.343 0.317 0.263 0.236 0.997 0.469 0.697
16 1.271 2.245 6.444 1.717 1.544 0.619 0.269 0.235 0.216 0.859 0.443 0.664
17 1.255 2.019 12.750 1.467 1.043 0.419 0.893 0.227 0.212 0.816 0.461 0.629
18 1.110 1.831 8.564 1.483 0.761 0.332 0.987 0.295 0.759 1.187 0.476 0.619
19 0.980 1.622 3.902 1.311 0.635 0.293 0.543 0.830 0.681 1.205 0.482 0.621
20 0.925 1.488 6.169 1.148 0.592 0.277 0.369 0.386 0.421 2.599 0.452 0.604
21 3.514 1.445 12.250 1.055 0.553 0.265 0.342 0.285 0.335 3.111 0.435 0.613
22 8.030 1.392 9.301 1.051 0.508 0.254 0.328 0.259 0.269 2.854 0.442 0.592
23 12.110 1.282 13.050 1.202 0.490 0.244 0.274 0.241 0.287 4.697 0.439 0.538
24 9.565 1.093 5.937 1.309 0.455 0.239 0.269 0.235 0.394 4.431 0.419 0.557
25 6.813 0.997 3.703 1.603 0.437 0.244 0.228 0.222 0.288 2.788 0.483 0.948
26 10.370 1.620 2.618 1.917 0.428 0.238 0.221 0.344 0.441 4.041 0.552 0.824
27 10.830 1.728 5.512 1.557 0.408 0.246 0.216 0.281 0.723 3.510 0.506 0.736
28 4.588 7.206 9.916 1.378 0.417 0.238 0.221 0.244 0.482 1.958 0.486 0.750
29 3.146 4.100 3.842 0.397 0.284 0.206 0.232 1.426 1.469 1.584 0.971
30 3.171 2.778 2.446 0.366 0.254 0.230 0.291 1.576 1.218 3.375 0.852
31 3.359 2.260 0.344 0.200 0.433 1.032 0.660
Average 5.346 5.181 5.506 2.642 0.984 0.308 0.347 0.476 0.422 2.799 0.688 1.458
Lowest 0.925 0.997 1.334 1.051 0.344 0.238 0.200 0.174 0.212 0.553 0.419 0.538
Highest 14.350 13.830 13.050 9.080 3.833 0.619 0.987 2.167 1.576 14.360 3.375 6.484
Monthly total
(million cu m) 14.32 12.53 14.75 6.85 2.64 0.80 0.93 1.27 1.09 7.50 1.78 3.91
Runoff (mm) 103 90 106 49 19 6 7 9 8 54 13 28
Rainfall (mm) 111 110 120 77 37 18 61 89 85 151 36 28
Statistics of monthly data for previous record (May 1970 to Dec 2000----incomplete or missing months total 0.1 years)
Mean Avg. 4.009 2.675 2.056 1.703 0.970 0.618 0.347 0.347 0.617 1.800 2.464 3.275
flows: Low 0.528 0.613 0.418 0.272 0.223 0.131 0.138 0.078 0.161 0.150 0.167 0.492
(year) 1992 1998 1993 1997 1976 1976 1976 1976 1971 1972 1978 1971
High 10.770 9.828 4.413 3.829 3.313 3.054 1.274 1.618 5.443 11.580 10.030 7.022
(year) 1988 1990 1981 1983 1981 1971 1991 1985 1974 1987 1974 1993
Runoff: Avg. 77 47 40 32 19 12 7 7 12 35 46 63
Low 10 11 8 5 4 2 3 2 3 3 3 9
High 207 171 85 71 64 57 25 31 102 223 187 135
Rainfall: Avg. 87 54 61 55 53 56 46 56 74 90 87 89
Low 16 7 7 4 3 8 6 2 13 9 15 19
High 183 138 127 125 115 147 103 155 201 224 209 167
Summary statistics Factors affecting runoff
2001
For 2001 For record As % of $ Augmentation from effluent returns.
preceding 2001 pre-2001
Mean flow 2.168 1.737 125
Lowest yearly mean 0.589 1973
Highest yearly mean 2.845 1974
Lowest monthly mean 0.308 Jun 0.078 Aug 1976
Highest monthly mean 5.506 Mar 11.580 Oct 1987
Lowest daily mean 0.174 1 Aug 0.051 8 Aug 1983
Highest daily mean 14.360 8 Oct 64.770 2 Oct 1993
10% exceedance 6.288 4.053 155
50% exceedance 0.858 0.587 146
95% exceedance 0.235 0.151 156
Annual total (million cu m) 68.37 54.82 125
Annual runoff (mm) 492 394 125
Annual rainfall (mm) 923 808 114
{1961-90 rainfall average (mm) 806}
Station and catchment description
Asymmetrical compound Crump profile weir (crests: 4.0m and 6.0m broad). Crest tapping not currently used; over-estimation
of high flows (structure drowns frequently); but post-1995 flows truncated at c15 cumecs. Stilling well leakage can
influence water levels. Limited impact of artificial influences on runoff - small net augmentation due to sewage
effluent, which can significantly affect low flow patterns.
Principally an impervious (Weald Clay) catchment - mainly rural but includes Horsham.
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