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The structure of the return laser altimeter pulse is partly a result of the reflectivity of plants near the canopy top ( Fig. 5.1a , Fig 5.1b , and Fig 5.1c ) . The physical variables that determine runoff and sediment transport include climate , vegetation , soil , and slope . The good news , however , is that the main differences between grocery store pears and those we can grow here are size and fresh fruit keeping ability . Presumably , changes in snow / ice surface conditions , including snowfall , blowing snow , melting , and other temperature-related changes , affect the scattering and reflectivity of the surface . For now , however , sanitation ( promptly removing and destroying fallen fruit and leaves ) , cultural practices designed to promote optimum tree health , and perhaps occasional physical or chemical intervention are all that is normally needed to obtain a satisfying crop in most years . Infected glads may have thin , weak , yellow leaves , and the flower spikes may be twisted and deformed , while the flowers remain green . If physical methods do not provide adequate control , use a residual insecticide such as carbaryl ( Sevin ) or permethrin ( Eight ) . How can its ascent be tracked to further our understanding of the magmatic plumbing system , the physical and chemical pro-cesses that result in eruptions , and provide advance warning that an eruption may be imminent ? Consequently , it is one of the most useful physical measurements we can make to improve estimates of earthquake potential . Although earthquakes were already treated in an earlier section , we note here that detailed geomorphic studies using primarily topographic data have yielded significant advances , including the location of active faults , especially where these faults are buried . This issue has been covered by several recent reports ( e.g ., Burke and Dixon , 1988 ; Mueller and Zerbini , 1989 ; Rundle , 1990 ) and we will not reiterate it here , save to update or refine some of the requirements for topographic data for the specific studies discussed below . Errors in the DEM contributed about 9 mm of noise to the coseismic offset estimates obtained by Massonet et al. ( 1993 ) , where rms vertical errors in the topographic data were estimated after the fact to be about + / -24 m. Systematic errors in selected areas can , of course , be much larger than this . The DEM for the Mojave region is representative of digital topographic data in existence for most of the Northern Hemisphere ( 90 - by 120-m pixels , nominal vertical accuracy of 30 m ) ; unfortunately , much of it is classified and not available for scientific research . On time scales of years to decades , meter-level or greater changes in topography may occur , but these are not the primary " observable " for many of the studies described here . In recent years , however , incidents of the disease have been reported from Illinois , Iowa , and Michigan , as well as here in Minnesota .