WMOD00519

• Project Name: Weather Modification Programme - Precipitation Enhancement Project
• Title: A Review of the Hydrological Aspects of Evaluation of Precipitation Enhancement
• Date: 5/1/1977
• Weather Modification - Doc Type: Report

11

53. Estimates of' hydrological response accurate enough to be useful in making
decisions about the degree of hazard involved in a particular set of circumstances
may be derived from mathematic8.1 modelling techni.ques. The availability of high-
speed digital computerB has encouraged many investigators to attempt simulationc of
the behaviour of a wide array of drainaee basins. The Stanford Watershed Hodel
(Crawforo, and Linsley 1 1966) 1 for example 1 converts input of hourly rainfall,
potential evapotranspiration, and watershed parameters into detailed outputs of
flo"" storage, and evapotranspj,ration. . Its use to assess the hydrological conse-
quences of precipitation enhancement is discussed. by Lumb and Linsley (1971).
'l'heir conclusions "rere that increases in anllual streamflow' resulting from precipi ta-
tioD augmentation varies from 0 to 106 per cent of the a(lded precipi ta tion for that
~ear; that in no case does a 10 per cent increase in rainfall result in more than
an 8 per cent increase in actual evapotranspiration; that increases in the mean of
the annual series of peak flo\'lS for the three watersheds studied ranged from 18 to
76 per cent and increases in the standard devia. tion ranged from 0 to 5,8 per cent;
and that not all of the runoff from the add~~d precipitation can be put to beneficial
use.

VIII

\.,rATERSHFJ) CHAH.ACTERISTICS DESIR.A:BI,E FOn PRECIPITATION ENRANCEHENT EVALUATION
:BY STREArlli'LOI'[ JiliD OTHE...l1 HYDROLOGICAL ME~ANS

54. Although \,ratershed characteristics have been mentioned earlier in connexion
with various aspects of the evaluation process, the folloHing discussion summarizing
characteristics of the target "\'Tatershed which contribute most directly to successful
evaluation by hydrological means may be a fitting way to conclude this review.

55. Perhaps the most basic requirement in that the watershed be adequately
gauged, preferably by recording stream gauges. If' there are diversions or storage
reservoirs in the watershed, these too should be adequately gauged. Adequate
gauging of a strea.m or reservoir can be said to exist. .Then t.he probable error of
the measurement is 5 per cent or less. If the topography of the basin is such as
to result in part of the precipitation occU2'rip~ a.s snowfall, the portions of the
basin in "Thich snowfall accumulates should be separately gauged,

'56. A long period of streamflow observations taken prior to the onset of
precipi ta tion enhancement efforts is desirable. Just hO"\1 many years of record con-
stitute a sufficiently long record depends heavily on the variability of the stream-
flow, but it would be unusual for a period as short as 10 years to provide a stable
basis for estimating expected flow during the seeded period. Certainly, 15 years
would be better than 10, 20 would be better than 15, and the fel;ler changes that have
occurred in the watershed during the period of record the better. Changes in mana.ge-
ment practices can prOduce changes in rillloff that are comparable to the increments
to be expected from cloud-seeding,

57. There should be as high a correlation as possible bet"\'leen available
precipitation records and the streamfloH records for comparable periods of time.
The less serial correlation exhibited by successive intervals of streamflo"T, the
more valid will be the statistical tests which assume independence of successive
events. Deep soil mantles capable of storing large amou..1'Jts of water temporarily
work against attainment of low serial correlations. The portion of the "\'later avail-
able for runoff \-lhich infiltrates the surface should be able to move promptly to
the stream rather than become part of a slo"\vly moving groillld-water bodJ'.

58. A compact basin shape contributes to a more homogeneous exposure to
precipi tation and reduces the opportunity for variation in the hyclrological response
to precipitation events. The existence of neighbouring basins with good streamflow
records has been an important factor in past evaluations. Annual records of 1'10"1
have provicled c:l.'edj,blo h;ydl'olog:i.cal evaluations in the past, but a.veraging over
shorter l)eriods should be considerod where hydro graph response to storm events permits.