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1 , ... 1~8 E.J. Bruw... W.E. &rJtlr,IJ.....--IufH,rtlrolofY!l6t/'J99J 1J7-1-U " EJ. 8m...... W,E. S.mIJlryIJoumalfJ/H_wlroloV216//999) IJ7-/4-I 1)9 , Fii'.I.o...ofIbcUlllmUsund.aupacoasuuctedllwUILi...,.. ,;~'oiWaikalo. as....... in lhe field withoulcrbolw-iQ,. 1M anxbed bOnlc !.n\'CI 10 COlkcl walefiDlllecvcl1Iof,JIlrc<>>'crl\ow. source. A strip of X-ray film in an opaque plastic CQ\"elope is attached to lhe outside of me gauge wall, and the ftoal rises upwards wilb waler level in the gauge during rainfall cvcn~. RainIess periods are identified on the developed film as horizontal lines, Cfcatcd whentbc f10al remains al the same level fora sutficienl time for the film 10 accumulate lhc effect of the radiation passing through the gauge wall. The magnitudes of individual rainfall events arc derived from the vertical intervalsbelwcenconsccutivchori- lonlallines onlhe film. In effect, the gauge crealcsa stratigraphic column of the rainfall events althe loca. tionconccmcd. The tenn "rainfall event" is defined here in a prag- matic way as two identifiable horizontal lines marking the !>tart und end of the event. Thus defined. the ability of the stillstand gauge to resolve a given period of rainfall as an .' event" depends on the combined effect of the: intensity of the marking process and the gauge venical exa,.eration (gauge orifice to gauge cross section mhO). The gauge crou section relates 10 storage capacity. so gauge height must increase with vertical exaggeration to avoid frequent field visits foc film replacement and gauge emptying. LOCalities experiencing particularly inlc::nse rainfalls may require gauge venical t<<luction rather than exauemtion (orifice: crosssecrion ratios < 1.0). The stills land gauges d.:veloped at lhe University of Wllikalo for field trials were designed with a venical ex.uggemlion of8.0, coupled with a 1.5 m height (Fig. n. This proved a reasonable compromise between event resolution acclllllCY and the need for field visilS to replace the film. Rainfall event resolution of the gauges is about 0.5 mm. given sufficient stillstand time before and after the event. 'The mdiatiOll source installed in the flOllt was Cobalt 57, a gamma emitter at 0.122 MeV and half- tife of 271 days. 1bc combined effect of venical maa:nification, radioactive source strength. and film sensirivjry were .such that .. recognisable horizontal line on the film required about 6 h of static waler level in the gauie. That is. the gauges recorded those rainfall events which were bracketed by two rain-free periods of at least 6 h duration. We found it best to develop the film ourselves to optimise me resolution of the stillstand lines. The film used had an operational temperature ranie of - 10"<: to 3S.C wh.ich was sufficient for OLlt purpose. It would be desirable to check manufacturers film specifications for stillstand gauges intended for extreme temperalUtC environments. The half-life of the Cobalt 57 radiation source WllS suitable for our field trial over a few months, allhough the marking time did increase somewhat as the radia- tion flux declined. Pennanent stillstand gauge networks would probably be best served by II different radiation source with a long half-life to give consis- tency in the marking procesS. This would also reduce on-Boing costs of replacing the radiation source. The nature of the radiation source has an influence on event resolulion, for a given degree of gauge verti- cal exaggeration. For example. beta radiation is more readily shielded than gamma radiation and thus can creale sharper lines during rainless periods. We found that Thallium Z04 (a beta emitter) gave better line " " E oS 14 .~ ../~ I~~~ (~. ; ! =:Y , . ! '-,i l;'f' , . ,.; ;) Jl ~ B C TIme (hr) ~=;:=~;::~s A fi,. Z. ~alleveDts as recorded by a lippi"l buckel,au,e aDdllw-e sliUSWldraUle.lA. B.C). Gau,e A and lhetippinlbucl<el were l<>Caled .l( \he ume sile. Gill..,' B alSdCY<Cfe wilbiD 3 Jun otlbe tippiD,bu.ckel'3uJe.1bc: linl:iDI' liDe,delUllelbe eslimaled.p;uia! c~I..u(lm lIfl/le I1liDfaUe,ellli. resolution Ihan the ulitised Cobalt 57. but !he fonner was unavailable 10 us at the time of the study. Safety aspects arc of imponance when placing any radiation source in Ihe environment. Each of our gauges was enclosed within a plastic outer housing which served as a radiation barrier. It was rccogni!>ed that radiation could enter the environment if the radio- active source somehow contaminated the gauge " .. c .. a: ~ " ... "3 E ~ o 10 . water, and there: W;J.S g~uge overflow. To guard against this possibility. overflow bottle!> were installed on all gauges. All the gauge water wa$ routinely checked for radioactivity but none was delccled. The raw material cost for each iauge was about S30. including film and development chemicals. Film costs were reduced by slicing the original film into Ihin strips 9 mm wide. The total dollar Cosl could