041019 Arun at Alfoldean 1999
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 2.234 1.428 3.612 0.791 0.453 0.290 0.317 0.171 0.208 0.910 0.804 1.003
2 3.717 1.299 5.989 0.673 0.446 1.858 0.253 0.177 0.226 0.972 0.841 0.923
3 12.690 1.227 8.232 0.618 0.415 0.712 0.240 0.219 0.201 0.832 0.634 0.790
4 7.749 1.178 5.667 0.556 0.405 0.498 0.225 0.547 0.191 0.707 0.606 0.658
5 5.106 1.107 2.705 0.544 0.451 0.544 0.271 0.496 0.201 0.517 1.485 0.550
6 3.330 1.217 1.992 0.575 0.381 0.524 0.243 0.296 0.201 0.449 1.534 0.544
7 4.676 1.245 2.123 0.528 0.464 0.926 0.209 0.727 0.179 0.423 0.883 0.612
8 4.929 1.168 2.131 0.483 0.495 0.488 0.192 1.157 0.174 0.441 0.715 1.399
9 3.402 1.185 1.569 0.467 0.383 0.341 0.184 0.506 0.179 0.437 0.599 1.868
10 2.462 1.034 1.283 0.468 0.351 0.295 0.176 0.365 0.172 0.374 0.536 1.198
11 1.867 0.901 1.173 0.604 0.335 0.268 0.170 0.294 0.174 0.349 0.523 2.994
12 7.640 0.857 1.466 1.158 0.330 0.346 0.173 0.233 0.167 0.315 0.499 3.918
13 4.941 0.799 1.621 0.791 0.316 0.286 0.170 0.249 0.175 0.315 0.486 5.060
14 3.151 0.852 1.177 0.621 0.559 0.257 0.172 0.331 0.291 0.295 0.468 3.169
15 6.256 1.027 0.999 0.539 0.411 0.237 0.170 0.358 0.246 0.300 0.448 1.682
16 13.560 1.031 0.910 0.495 0.330 0.231 0.166 1.364 0.244 0.281 0.489 1.277
17 6.009 0.968 0.858 0.455 0.314 0.221 0.173 1.235 0.227 0.275 0.463 1.328
18 5.402 0.947 0.809 0.426 0.533 0.216 0.170 2.463 0.271 0.266 0.588 1.518
19 14.070 1.112 0.725 0.434 0.706 0.199 0.182 0.808 0.881 0.275 0.531 1.976
20 13.000 1.087 0.692 3.966 0.676 0.223 0.216 0.417 2.615 0.270 0.500 1.362
21 6.184 1.058 0.992 3.173 0.558 0.210 0.178 0.295 1.179 1.115 0.474 2.578
22 3.837 0.921 0.822 1.342 0.372 0.209 0.159 0.253 0.619 3.503 0.486 4.733
23 3.032 0.799 0.757 1.062 0.315 0.197 0.161 0.248 1.955 5.205 0.467 6.953
24 2.637 0.937 0.665 0.913 0.306 0.193 0.153 0.241 5.878 9.450 0.450 13.870
25 2.987 0.819 0.666 0.779 0.277 0.199 0.147 0.266 5.875 3.211 0.459 13.310
26 4.447 1.030 1.358 0.800 0.263 0.178 0.154 0.243 2.134 1.432 0.589 14.410
27 3.606 3.381 0.978 0.642 0.251 0.225 0.173 0.234 4.881 0.945 0.851 7.699
28 2.487 1.889 0.713 0.549 0.237 0.546 0.178 0.212 2.819 0.788 0.755 4.110
29 1.726 0.695 0.516 0.227 0.693 0.178 0.204 2.651 0.699 0.889 2.863
30 1.626 0.996 0.481 0.279 0.361 0.174 0.205 2.140 0.854 1.114 9.216
31 1.517 0.936 0.272 0.170 0.223 0.810 8.029
Average 5.170 1.161 1.784 0.848 0.391 0.399 0.190 0.485 1.245 1.194 0.672 3.923
Lowest 1.517 0.799 0.665 0.426 0.227 0.178 0.147 0.171 0.167 0.266 0.448 0.544
Highest 14.070 3.381 8.232 3.966 0.706 1.858 0.317 2.463 5.878 9.450 1.534 14.410
Monthly total
(million cu m) 13.85 2.81 4.78 2.20 1.05 1.03 0.51 1.30 3.23 3.20 1.74 10.51
Runoff (mm) 100 20 34 16 8 7 4 9 23 23 13 76
Rainfall (mm) 120 35 45 42 28 70 6 107 141 66 34 137
Statistics of monthly data for previous record (May 1970 to Dec 1998----incomplete or missing months total 0.1 years)
Mean Avg. 4.049 2.750 2.101 1.669 0.947 0.629 0.340 0.345 0.569 1.691 2.393 3.148
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. 78 48 40 31 18 12 7 7 11 33 45 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. 87 54 63 53 51 57 46 54 70 86 85 85
Low 16 7 7 4 3 8 12 2 13 9 15 19
High 183 138 127 115 115 147 103 155 201 224 209 167
Summary statistics Factors affecting runoff
1999
For 1999 For record As % of $ Augmentation from effluent returns.
preceding 1999 pre-1999
Mean flow 1.465 1.716 85
Lowest yearly mean 0.589 1973
Highest yearly mean 2.845 1974
Lowest monthly mean 0.190 Jul 0.078 Aug 1976
Highest monthly mean 5.170 Jan 11.580 Oct 1987
Lowest daily mean 0.147 25 Jul 0.051 8 Aug 1983
Highest daily mean 14.410 26 Dec 64.770 2 Oct 1993
10% exceedance 3.722 3.914 95
50% exceedance 0.612 0.572 107
95% exceedance 0.174 0.149 116
Annual total (million cu m) 46.20 54.15 85
Annual runoff (mm) 332 390 85
Annual rainfall (mm) 831 791 105
{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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