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<br />employed. Examples of such procedures include the hydrologic record extension techniques <br /> <br />described by Searcy (1960), Hirsch (1982), and Alley & Burns (1983). Hydrologic simulation <br /> <br />modeling or water budgeting techniques can also be used to synthesize a hydrologic record <br /> <br />(Linsley et al. 1982). <br /> <br />, ,~ .' <br />.'l>'_.?' <br /> <br />Accounting,for Climatic Differences <br /> <br />"'-;' '-roT, <br /> <br />- >~:,-,;~--- ' <br />~~-.".". <br /> <br />Climatic differences,between the;pre",andpost-impacttime periods obviously have the";;;$:le' <br /> <br />>.;~:~r ,._',~' <br />:',-:;,=; <br />'_.s_c''-.' <br />;':?:~."J:;.' ': <br /> <br />potential to substantially influence the outcome of the.IHA analysis. Various statistical>: ...iT <br /> <br />techni~ues can. be used to test for climatologic differences in the hy~rolog\c.data to be.,. "~~ <br /> <br />compared. When the IHA analysis is to be based upon actual hy:drologic measurements rather:..-}.\\; ;', <br />.-- . -'..-.?i~~~:tt:; <br />than esti~tes p~uced from simulation modeling, a reference site or set of sites uninfb.Q,r ..tl;;:{};;~;> <br />. _"~~_:~'?T\~, <br />by the human alterations being examined can be used as climatic controls (Alley & Bumsz;;:.;i '-{i; <br />,'- ~.., '., "'_ij;:!:;~:;,::: <br />1983). For example, if a streamgauge exists upstream of the reservoir impacts being.eV"'lmrIPrl <br /> <br />at a study site, a statistical relationship between stream flows at the study site and this upstream <br /> <br />reference site can,betdeveloped;from.synchronous~pre."'(jam data sets for the two sites~ This:J.". <br /> <br />relationship can then be used to reconstruct (synthesize) the streamflow conditions thatwoulW;; <br /> <br />have been expected at the study site during the post-imp~ct time Reriod in -absence of. the.:;..)i!!...l.., <br />.' - - - . ~:~--'~~l~i~,: <br />reservoir. In this situation, the IHA method is then used to compare the;measuredpost'iimpact"':'~;;; <br />". - - " ' .:. ',-,>..,,,:~~~-'>~_'.o' <br /> <br />conditions with synthesized estima.tes of the. unimpacted conditions f()r the same time period. <br /> <br />. .,., ,~-. <br />~+ji;'<~::<'" <br /> <br />-<~.~;,~+ -~~': -.' <br /> <br />Alternatively, a time series of observed impact-control differences that spans the time of{ ;"im <br /> <br />perturbation at the impact site can be used to assess hydrologic impacts (e.g., Paller euh:. ~:.)(f.' <br /> <br />, ..?')I' <br /> <br />1992); this is the basis for the Before-After-Control-Impact-Pairs design suggested by Stewart- <br /> <br />15 <br />