(0
5
C allibration of sim u lato r b o ard #1, 23/3/96, 4 mm in s e rt 7 0 6 0 5 0 4 0 y = 1 .4 9 1 3 X + 2 4 . 4 0 4 R?=0.8862 3 0 20 10 0 0 5 10 1 5 20 2 5 3 0
Difference in hydraulic head (mm)
I
Callibration of sim ulator board #1,23/2/96, 3nfm insert 5 0 4 0 3 0 y = 0.9502x + 9.1661 20 Ff = 0.9562 10 0 0 10
20
30 40 Difference in hydraulic h ea d (mm# C a llib r a t io n o f s i m u l a to r b o a r d # 1 ,1 5 /3 /9 6 , 3 m m i n s e r t 7 0 6 0 5 0 4 0 y=0.6063x+ 14.678 3 0 = 0.974 20 10 0 0 20 40 60 80 100 D ifference in h y d ra u lic h e a d (mm )possibly be due to cleaning of the board in the interim, of up to 8 mm/hr difference at a difference of 40 mm between capillaries; this has potentially serious consequences if it were not discovered through monitoring changes in cahbration throughout the field season.
Even more serious a problem slowly became apparent as represented in Figure 4.16, the variability in calibrations over the course o f the field season, for actual simulations carried out under field conditions. This is not apparent in (a), cahbrations with the 2 mm insert, probably because of the few simulations carried out with this aperture. However, with (b) the 3 mm insert, the results reveal substantial overlap in error bars in the rainfall intensity supposedly
corresponding uniquely to differences in manometer readings which are widely separated. As a result, for example, one can barely separate, with confidence, the actual intensities of the target intensities o f 33 and 53 mm/hr, due to a variability of +/- approx. 8 mm/hr for the former and +/-
10 mm/hr for the latter. Whilst the statistical probability of the lower target intensity erring on the ‘positive’ side of the average intensity whilst the higher target intensity errs on the negative, such that the error bars overlap, is only 0.25 on the basis of this data, it is still a major cause for concern.
(4.16.2)
Calibrations of board # 2
The calibrations with the 3 and 4 mm insert (which provided the necessary range of intensities while providing a low graphing gradient; this demonstrates the individual hydrauHcs o f each board; for board # 1 the 2 and 3 mm inserts were preferable) are presented in Figure 4.17. With both inserts the line of best fit is logarithmic, whilst with both inserts for board # 1 it is linear, which confirms the point above about the individuahty of each board / system and hence the importance o f re-calibration for any change in the system (for example, changing the water datum in the form of the height of the jerry cans above the board), which can be time consuming (one day). The fit is tight around the mean even with three replicates (4 mm insert).
Figure 4.18, the calibrations under field conditions, reveals, as with board # 1, little variability in the case of only a few replicates per target intensity / difference in capillaries, but, again as with board # 1, with more replicates, the variability becomes much greater. For example, at a difference in capillary of 15 or 16 mm, the corresponding rainfall intensities in the scatterbox
hydraulic head maintained on the manometer, field season # 1, board # 1
a) Simulations carried out in the field over the course o f the first field season, using the 2 mm insert reveals little difference in intensity for a large change in hydraulic head.
b) Using the 3 mm insert, with which more runs were carried, due to variability in intensity between multiple simulations at a given difference in hydraulic head, it is not possible to unambiguously discriminate, for example, between a 10 and a 25 mm difference in hydraulic head, due to the overlap in corresponding intensities. This is in stark contrast to the tight clustering around the line o f best fit achieved for the calibrations carried out at the field station.
Correspondence under field conditions between hydraulic head and intensity, over the course of the field season, simulator #1,2 mm insert
4 5 1 4 0 JC y = 0 .1 5 7 2 x + 8 .9 7 3 2 = 0 .8 6 7 3 2 5 0 50 100 150 200 2 5 0 D iffe re n c e in h y d r a u lic h e a d (m m )
C o rresp o n d en ce u n d er field co nditions betw een hydraulic h ead a n d intensity, over th e c o u rs e of ttie field s e a s o n , sim ulator # 1 ,3 m m insert
80 70 60 50 y = 0.4633x + 21.214 F^ = 0.8687 40 30 20 10 0 20 40 60 80 100 120
Figure 4.17 Field station calibrations of board # 2 , 3 and 4 mm inserts
a) 3 mm insert. Best fit (r2 = 0.98), is logarithmic curve, unlike board # 1, in which case the best fit is linear. The reason for this difference is not known.
b) 4 mm insert. Same comments as for (a). Allows a wider range o f intensities (here to 80
m m /h r^
C a lib ra tio n o f s im J a t o r b o ard # 2 ,1 5 /3 /9 6 ,3 n m in sert 60 50 y=20.05Ln(x)-27.792 R^ = 0.9787 10 0 0 10 20 30 40 50 80 70 Différence in h y d ra iiic h e a d (rrm )
C allibration of s in rü a to r b o ard # 2 ,2 3 /3 /9 6 ,4 n m in sert
E50 y=32377üT(x)-28.361 if = 0.9856 w 40 - 30 0 5 10 15 20 25 30
for all simulations in field season # 1, for 3 and 4 mm inserts a) Using 3 mm insert, a tight relationship is apparent (r2 = 0.965)
b) Using 4 mm insert, a much greater variability in measured rainfall intensity for any given hydraulic head setting on the manometer is evident. This is likely partly due to the greater number o f ‘replicates’ at each setting, and the larger change in intensity for a given change in head with a larger aperture insert.
Callibration, u n d e r field conditions, betw een hydraulic h e a d a n d intensity, b o ard # 2 ,3 m m in sert y = 0.9e64x + 7.6589 FP = 0.965 > 3 0 £ 20 Difference in hydraulic h e a d (mm)
C allibration u n d er field co n d itio n s b etw een hydraulic h ead a n d intensity, board # 2 ,4 m m in sert
100
y =2.7854x+12.262 R^ = 0.7732
range jfrom 44 to 69 mm/hr! In order, therefore, to separate actual intensities without ambiguity, one would have to, for example, set a target intensity o f 55 mm/hr and accept a window of about 35 mm/hr - 55 mm/hr - 85 mm/hr within which another target intensity (where intensity is a treatment) is not permitted. The greater intensity ‘buffer’ at the higher end is the result of greater variability in intensity, although this may be the result o f one (out o f four replicates) outlier.
Figure 4.19, a comparison of the lines of best fit though the field station and field season calibrations respectively reveals the consequences o f the difference in fit between these two data sets. The logarithmic response o f declining marginal increase in intensity for every unit increase in hydraulic head, which characterises the field station calibration, is not true o f the field season results. The reason for this is not known, but the consequence would be large errors (in addition to the intensity variability around the line o f best fit within each data set) in the apparent intensity of the field season simulations at the low and high target intensities, as illustrated in (a). This effect has not been reported, to one’s knowledge, in the literature, leading one to suspect that the calibrations whenever the simulator was first set up were assumed by others to hold true over time, which is manifestly not the case at least in this experience, and with serious implication in terms of the apparent effects of rainfall intensity on erosion or crusting or infiltration. The design of this widely used simulator should be modified to allow for continuous monitoring of the relationship between hydraulic head and intensity.
(4.17)
Conclusion; implications of results of calibrations
To summarise briefly, it is believed that the investment in time o f some four weeks o f calibrations over the two field seasons plus ongoing calibrations over the course o f the field seasons yielded very significant findings about the variability in the calibration between a) the difference in the height of the water in the two capillaries of the manometer board and b) the intensity of the rainfall produced under the simulator board, the central principle around which this simulator is designed. As apparent from the amount of time required to identify this phenomenon, it is tempting to calibrate once, at the begiiming o f the field season, or to rely on calibrations fi"om another user, and spend one’s time instead actually using the simulator. This, however, would be a false economy; it is known fi*om the literature review that rainfall characteristics have a major,
Comparison of lines of best fit through the calibration points between rainfall intensity and hydraulic head at the field station and during field work, board # 2 With both the 3 and 4 mm inserts the line of best fit through the field data is a straight line, whereas for the field station calibrations the line o f best fit is logarithmic. Whatever the reasons for this, an important implication o f this is that a calibration established at the beginning o f the fieldwork cannot be relied upon uncritically during fieldwork. This discrepancy is greatest at the lowest and highest settings o f hydraulic head / intensity; at low intensities (see (a)), the resulting error in apparent intensity can be well over 100%, and at high intensities the difference, for example, between 53 and 68 mm/hr.
Comparison between lines of best fit for Field and Station callibrations of hydraulic head with intensity, board # 2 , 3 mm insert
♦ Field callibration o Station callibration Linear (Field callibration) Log. (Station callibration) 2 0 30 4 0 50
Difference in hydraulic head (mm)
Comparison of lines of best fit for Field and Station calibrations of hydraulic head and intensity, board # 2 ,4 mm insert 120 100
!
♦ Field calibration □ Station calibration Si c — Linear (Field calibration) — Log. (Station calibration) 0 5 10 15 20 2 5 30 35if not dominant influence on erosion, crusting, sealing, infiltration and runoff, and rainfall intensity is explicitly used as a treatment in many rainfall simulation studies. In addition, the widespread use of this design over the past two decades mean that the findings o f the present research, if accurate, imply the necessity of re-examining many of the results of studies relying on this simulator design.
In terms of the impact on the present study, this problem has been mitigated by the fact that the
actual intensity of each simulation was measured, in the first field season, over the first five minutes (after allowing five minutes for the system to stabilise, as indicated from the cahbrations) preceding the simulation and the last five minutes after termination of the experiment. In response to the findings over the first field season, the target intensity continued to be set by way of the manometer board, but the actual intensity was monitored on a continuous, real-time and spatially distributed manner as described with respect to the design of the 3.5 m simulator and which, it is argued, resolves the problem of errors of analysis but does not alleviate comphcations in
comparability between treatments resulting from deviations from a target intensity. Furthermore, if rainfall intensity is to be used as a treatment, then the target intensities specified must be separated by an amount greater than the potential overlap o f error bars of actual intensity around these target intensities. Rough values of these error bars have been provided by the present research, but must be determined on a system-by-system basis due to differences in board and system hydraulics, as indicated by other calibrations carried out in this study.
(4.18)
Protocol used in the rainfall simulation
Table 4.5 details the parameters chosen for assessing hydrological characteristics and the bases on which comparisons between simulation runs will be made.
The issue of the multiplicity of possible parameters with which to characterise the hydrological behaviour of the surfaces assessed is returned to in the results of the simulations from the first field season, and the parameter-dependence of the apparent relative runoff potential of the simulation sites will be evident. Figure 4.20 presents the principal ‘treatments’ (changes in surface or rainfall or both) forming the basis of the research design, although antecedent moisture