041019 Arun at Alfoldean 1996
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 1.501 0.558 1.198 1.025 0.432 0.287 0.192 0.182 0.201 0.480 0.406 1.760
2 1.426 0.548 1.123 0.781 0.435 0.276 0.204 0.175 0.213 0.265 0.420 1.472
3 1.073 0.509 1.094 0.640 0.403 0.290 0.194 0.166 0.145 0.240 0.725 6.925
4 1.252 0.487 1.023 0.632 0.351 0.269 0.196 0.160 0.133 0.246 4.522 3.415
5 2.071 0.485 0.954 0.610 0.348 0.264 0.189 0.155 0.146 0.196 0.958 1.858
6 4.340 0.519 0.877 0.642 0.362 0.250 0.286 0.192 0.142 0.184 0.859 1.349
7 7.108 0.468 0.837 0.632 0.348 0.243 0.285 0.205 0.144 0.190 0.780 1.148
8 8.183 0.476 1.922 0.612 0.312 0.223 0.223 0.169 0.142 0.199 0.646 1.008
9 13.910 3.699 3.426 0.586 0.301 0.231 0.190 0.187 0.147 0.453 0.605 0.935
10 6.640 5.042 2.134 0.571 0.310 0.219 0.183 0.340 0.143 0.255 3.152 0.870
11 4.992 3.646 1.526 0.581 0.308 0.210 0.176 0.467 0.138 0.214 1.454 0.793
12 4.184 12.160 1.245 1.372 0.306 0.201 0.179 0.327 0.142 0.199 0.933 0.750
13 2.978 4.366 1.058 1.690 0.302 0.231 0.179 0.251 0.164 0.188 1.224 0.709
14 2.295 2.357 0.932 0.946 0.309 0.181 0.170 0.195 0.161 0.264 0.706 0.633
15 1.774 1.556 0.853 0.741 0.327 0.169 0.171 0.175 0.163 0.291 0.533 0.619
16 1.504 1.476 0.775 0.700 0.479 0.169 0.159 0.169 0.173 0.209 0.447 0.617
17 1.281 1.279 0.697 0.638 0.553 0.186 0.153 0.158 0.153 0.192 0.587 0.664
18 1.168 1.435 0.676 0.570 0.512 0.181 0.156 0.152 0.155 0.488 0.882 0.758
19 1.147 1.146 0.642 0.546 0.412 0.178 0.153 0.157 0.191 0.330 11.450 0.990
20 1.053 0.948 0.658 0.542 0.373 0.213 0.150 0.152 0.179 0.272 9.289 1.100
21 0.937 0.852 2.251 0.510 0.380 0.246 0.166 0.157 0.166 0.244 2.954 1.090
22 0.878 0.915 3.420 0.737 0.548 0.217 0.141 0.431 0.153 0.210 1.891 0.897
23 1.108 1.142 3.184 0.823 0.494 0.201 0.135 1.135 0.145 0.204 1.686 0.759
24 1.395 3.166 1.863 0.657 0.807 0.197 0.189 0.445 0.139 0.200 5.663 0.633
25 0.983 11.860 2.053 0.533 0.904 0.186 0.177 0.335 0.164 0.330 7.438 0.526
26 0.772 4.581 7.692 0.480 0.566 0.188 0.151 0.324 0.178 0.302 2.596 0.475
27 0.695 2.379 3.121 0.466 0.565 0.189 0.147 1.049 0.175 0.647 1.600 0.479
28 0.664 1.673 1.899 0.443 0.418 0.195 0.154 0.859 0.163 0.597 1.398 0.483
29 0.633 1.377 1.578 0.429 0.389 0.192 1.126 0.461 0.271 0.570 5.016 0.482
30 0.592 1.291 0.441 0.355 0.186 0.381 0.321 0.814 0.386 2.821 0.499
31 0.570 1.159 0.309 0.257 0.266 0.476 0.466
Average 2.552 2.452 1.715 0.686 0.426 0.216 0.220 0.320 0.185 0.307 2.455 1.134
Lowest 0.570 0.468 0.642 0.429 0.301 0.169 0.135 0.152 0.133 0.184 0.406 0.466
Highest 13.910 12.160 7.692 1.690 0.904 0.290 1.126 1.135 0.814 0.647 11.450 6.925
Monthly total
(million cu m) 6.84 6.14 4.59 1.78 1.14 0.56 0.59 0.86 0.48 0.82 6.36 3.04
Runoff (mm) 49 44 33 13 8 4 4 6 3 6 46 22
Rainfall (mm) 73 70 50 25 41 8 43 75 31 64 145 25
Statistics of monthly data for previous record (May 1970 to Dec 1995----incomplete or missing months total 0.1 years)
Mean Avg. 4.185 2.818 2.162 1.683 0.999 0.648 0.347 0.353 0.606 1.777 2.351 3.190
flows: Low 0.528 0.689 0.418 0.277 0.223 0.131 0.138 0.078 0.161 0.150 0.167 0.492
(year) 1992 1981 1993 1976 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. 81 49 42 31 19 12 7 7 11 34 44 61
Low 10 12 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 54 65 54 52 55 48 54 72 84 81 87
Low 16 7 7 4 3 12 12 2 13 9 15 19
High 183 138 127 110 115 147 103 155 201 224 209 167
Summary statistics Factors affecting runoff
1996
For 1996 For record As % of $ Augmentation from effluent returns.
preceding 1996 pre-1996
Mean flow 1.050 1.757 60
Lowest yearly mean 0.589 1973
Highest yearly mean 2.845 1974
Lowest monthly mean 0.185 Sep 0.078 Aug 1976
Highest monthly mean 2.552 Jan 11.580 Oct 1987
Lowest daily mean 0.133 4 Sep 0.051 8 Aug 1983
Highest daily mean 13.910 9 Jan 64.770 2 Oct 1993
10% exceedance 2.265 3.934 58
50% exceedance 0.488 0.577 84
95% exceedance 0.152 0.148 103
Annual total (million cu m) 33.20 55.44 60
Annual runoff (mm) 239 399 60
Annual rainfall (mm) 650 795 82
{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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