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OOf\71? soils. Thctl\'erallaverage is 1 inch or slightly more: the lower. drier sill's produu'! very Iillle. while the well est sites ma~' yield 3 or"\ inches. The putential for increasing stte<imno\\' by type conversion of chaparral is guud on favorable sihtS where precipitation averages 20 inches or more (Hibbert et 011. HI74). Thl'! key to increasin~ waler yield is the replacement uf dt!ep-rooted shruhs with shallo\\'+toolt!d grasses and forb!'! that use less waler. The ollsite inCft!8ses determined from ex- perimentallype conversions on small watersheds range from Ipss than 1 inch to mtire than 5 inches lfig. 1 ~)). The i1\"erage is 3.8 inches at 22 inches precipitation. Some discounting is in orriN hcfoft' extrapolating the ftlsearch results to larger areas where cunversion ma\" not be as intensive, con- tinuous, or as well maintained as on the experi- mental watersheds. Moreover. some of the in- creased llow may be lost to riparian \'egetation downstream before it reaches storage or points of use. Therefore. the a\'erage increase expected downstream from type cOl1\'ersion is estimated to be about hvo-thirds of the onsite increase, or 2.4 inches where precipilation is 22 inches (con- sidered average for treatahle chaparral). Other con- siderations will limit tht~ amount of type com'cr- sian to a fraction of the tolal acreaKe. Of the 1.8 million acres of chaparral on National Forest lands. more than 200,UOO acres are in wil- dt~rness and other special use areas. where conver- sion is not compatible \'vith pft~sent land use pol- icy. A much larger portion. nearly 40% of the re- maining chap<-lrral. is considered too dry and open (crown COVHr less than 30'''0) to be a KOOtI risk for waler vield impron~menl. Also. much of the chapar~al is nil excessi\'ely steep slopes. In a sludy of chaparral con\'ersion potential on National Forest lands in the Salt-Verde Basin above Phoenix. Ariz. IT. Brown et al. 1974). about 10% of otherwise treatable chaparral was on slopes steeper than fiO'....... which was the upper limit con- !'iitiered safe for conversion. Ilowever. some land managers fed that the 60% !'ilope criterion is too steep for practical field application, and that the upper limit should be 50%, or t!\'en ..HJ%. Sinct! approximately 40% of the chaparral is on slope!'i between 40U" and 60%. the steepness at which conversion call be safely accomplished is ex- tremel\' important. Approximately 20% less area would 'bt' available for conversion if the maximum operable slOlH' is lowered from 6001;, to 50%. and another 20% of acrt'agl' would he lost belween 50% and ..0% slope. A]so. substantial acrea!,:es may be excluded for treatment. or gi\'en low priority. be- cause of opt'rational rt'striclions or ~l.'{)graphic ]0- cation. such as chaparral on !'ilopc!'i of isolated mountain ranges. wht.rt~ an increast' in waler 8 . }' u = 6 . E T fealed E 0 . . 38 inch ;;; <; lncr~$e , < < 2 0 < 0 . Natural ~ 0 26 16 18 20 22 2' Mean annuat precipitation (Inches) Figure 19.-Average water yIeld from naluraland converted Chaparral Is a function of precipitation. Difference be- Iween lines Is aUribUled 10 Ireatmen! (Hibbert el al. 1915). wou]d he of questionable value. Therefore. buth the t acre in 5 estimate for treatable chaparral and the 2.4-inch water yield increase projp.clion proba- bly would be cunsidered th!t most optimistic po- tentials allainable by large-scale management ef- forts ill the chaparral. Further. use of the arithmetic a\'erage to dl>scrihe annual water yield potential can he misleading, bl~r:ause this vahlt' (2.4 indws in this casl~) will not hI! equalled or p.xcet~ded 511% of the years as would occur if precipitation and rUIloff were normalJ~' distributed. In Ihe chaparral of C(mtral Arizona, precipitation is less than averagl' about 58"/0 of the \lears. Since runoff increases exponentially with im:cipitation. yearly runoff amounts are skewed ever farther to the tir... side of the distribution curve. Wet years tend t~ yielti many tinws as much water as dr\' unes. Thus. l!\'en though precipitation Hxceeds th~ mean in nnl\' aboul4 \lears in 10. more than 80% of the water vield increases are produr:ed in these weller tha'n aVl!ragn :'.'ears. Storage facilities are imperative to get full benefit of in- creastld water \lields in wet vears. To the extent that downstream s-Iorage capacity is exclleded {which happened 7 times in lhe past 50 years in the Salt- Verde Basin above Phoenix) and willt'r is spilled. runoff increases from conversion would b(~ lost to the full extent of the release, assuming no further beneficial use could be made of the water down~ stream. ~ ~1ountain Brush ~lountain brush lands (fig. 201 are exh'nsi\'e only in the Upper Basin. where they are found on about 15