041019 Arun at Alfoldean 2000
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 3.862 0.814 3.401 0.511 1.322 1.416 0.275 0.246 0.299 2.318 4.356 14.390
2 2.966 1.802 2.044 0.631 1.106 1.203 0.277 0.253 0.243 2.556 13.220 7.626
3 5.863 1.225 5.539 2.143 0.964 1.046 0.254 0.948 0.200 1.165 8.601 4.526
4 6.365 0.985 2.694 6.258 0.920 0.882 2.958 0.490 0.186 0.813 3.959 13.230
5 3.571 0.896 1.748 2.814 0.815 0.800 7.781 0.320 0.177 0.792 9.403 7.456
6 2.674 1.067 1.456 1.274 0.695 0.796 1.177 0.267 0.203 0.672 13.630 6.269
7 2.191 3.576 1.330 0.979 0.720 0.691 0.593 0.245 0.172 4.175 11.570 10.150
8 2.145 3.358 1.182 0.843 3.365 0.593 0.411 0.524 0.180 2.900 5.577 12.940
9 1.703 1.867 1.042 1.038 3.873 0.944 0.411 0.384 0.182 8.768 3.529 11.640
10 1.442 3.767 0.947 0.771 1.960 0.656 0.682 0.286 0.185 13.490 2.534 13.980
11 1.332 2.494 0.873 1.657 1.188 0.518 0.597 0.243 0.167 13.990 7.352 11.720
12 1.324 3.916 0.780 7.321 0.953 0.468 0.379 0.222 0.164 13.080 10.290 14.490
13 1.863 2.644 0.733 6.178 0.820 0.440 0.383 0.209 0.165 4.713 4.174 14.420
14 1.520 1.887 0.730 3.632 0.690 0.416 0.327 0.237 0.157 2.678 2.977 7.851
15 1.271 1.781 0.630 10.580 0.622 0.404 0.275 0.241 4.750 1.805 2.384 4.611
16 1.140 1.798 0.594 3.973 0.560 0.389 0.247 0.265 1.549 2.787 7.015 3.529
17 1.056 1.501 0.573 6.305 0.510 0.359 0.238 0.234 0.583 2.270 3.502 2.707
18 0.986 2.007 0.547 6.269 0.576 0.326 0.229 0.263 1.819 3.475 2.779 2.612
19 0.913 1.508 0.521 5.696 0.505 0.311 0.239 0.217 8.105 2.765 2.939 2.455
20 0.868 1.180 0.519 7.935 0.571 0.417 0.210 0.204 3.972 7.958 2.452 2.290
21 0.832 1.201 0.535 4.467 0.908 0.346 0.199 0.218 1.204 11.410 5.794 2.005
22 0.927 1.119 0.505 2.871 0.573 0.308 0.186 0.193 0.704 5.034 7.958 1.801
23 0.855 1.045 0.485 1.902 0.645 0.303 0.184 0.176 0.520 4.844 9.356 1.664
24 0.799 1.498 0.572 1.859 1.176 0.284 0.190 0.162 0.661 4.017 5.191 2.479
25 0.749 3.293 0.537 3.339 1.338 0.280 0.192 0.149 1.198 2.921 9.771 3.938
26 0.669 1.795 0.506 4.166 7.202 0.278 0.194 0.191 1.508 2.644 5.573 2.538
27 0.635 1.509 0.501 2.377 8.345 0.271 0.187 0.175 2.407 2.545 5.161 1.925
28 0.715 1.733 0.455 2.603 13.940 0.249 1.717 0.163 5.128 9.739 6.689 1.597
29 0.987 7.547 0.488 2.879 7.540 0.251 0.685 0.162 2.516 12.430 5.626 1.422
30 0.828 0.480 1.662 2.950 0.254 0.383 0.171 2.255 14.420 6.354 1.177
31 0.723 0.468 1.707 0.263 0.185 9.168 5.551
Average 1.735 2.097 1.078 3.498 2.228 0.530 0.720 0.266 1.385 5.559 6.324 6.290
Lowest 0.635 0.814 0.455 0.511 0.505 0.249 0.184 0.149 0.157 0.672 2.384 1.177
Highest 6.365 7.547 5.539 10.580 13.940 1.416 7.781 0.948 8.105 14.420 13.630 14.490
Monthly total
(million cu m) 4.65 5.25 2.89 9.07 5.97 1.37 1.93 0.71 3.59 14.89 16.39 16.85
Runoff (mm) 33 38 21 65 43 10 14 5 26 107 118 121
Rainfall (mm) 26 70 19 125 115 17 93 46 135 222 206 147
Statistics of monthly data for previous record (May 1970 to Dec 1999----incomplete or missing months total 0.1 years)
Mean Avg. 4.087 2.695 2.090 1.641 0.928 0.621 0.335 0.350 0.591 1.675 2.336 3.174
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. 79 47 40 31 18 12 6 7 11 32 44 61
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. 89 53 62 53 51 57 45 56 72 85 83 87
Low 16 7 7 4 3 8 6 2 13 9 15 19
High 183 138 127 115 115 147 103 155 201 224 209 167
Summary statistics Factors affecting runoff
2000
For 2000 For record As % of $ Augmentation from effluent returns.
preceding 2000 pre-2000
Mean flow 2.642 1.707 155
Lowest yearly mean 0.589 1973
Highest yearly mean 2.845 1974
Lowest monthly mean 0.266 Aug 0.078 Aug 1976
Highest monthly mean 6.324 Nov 11.580 Oct 1987
Lowest daily mean 0.149 25 Aug 0.051 8 Aug 1983
Highest daily mean 14.490 12 Dec 64.770 2 Oct 1993
10% exceedance 7.585 3.910 194
50% exceedance 1.189 0.573 208
95% exceedance 0.185 0.151 123
Annual total (million cu m) 83.55 53.87 155
Annual runoff (mm) 601 388 155
Annual rainfall (mm) 1221 793 154
{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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