Figure 9 shows the physiographic provinces of the study area: the coastal plains, the foothills, the Temecula Valley, the mountains, and at the eastern edge, the desert.
The coastal plains are rolling to steep topography with fairly smooth terraces that support a natural cover of chaparral and grassland. In the narrow winding valleys, oak is the dominant vegetation. There are also introduced eucalyptus and other exotic trees. The unequal altitudes of terraces of the same age indicate geologic uplift as the major cause in forming the terraces. The lowest terraces have been cut by waves and have distinct cliffs or escarpments along their seaward edge. The elevation of the marine terraces ranges from nearly sea level to about 250m.
The coastal plains have the most equable climate of any area in the country. As with most of the study region, temperature and precipitation vary directly according to elevation, distance from the coast, and solar aspect. Generally, the temperature decreases and the precipitation increases with rising elevation. The mean annual temperature of the coastal plains is 15 Celsius, with a mean minimum temperature in January of 6C. The frost-free season is 280 - 360 days and the winter growing season experiences only light frost. Annual rainfall ranges from 0.3 - 0.4m, and 90% of this falls between November and April. Most soils are thoroughly moistened during this period, but little leaching occurs. Soil moisture is used up during rapid plant growth from early spring through June. Unless irrigated, the soils are dry by summer and soil moisture becomes the main factor limiting plant growth.
The foothills are a belt of narrow winding valleys and rolling uplands that have few very steep slopes. This zone lies between the coastal plains and the mountains. It is about 45km wide and extends in a northwest-southeast direction through Orange and Riverside Counties to the Mexican border. Chaparral, open woodland, and isolated areas of open grassland make up the typical plant cover. The elevation ranges from about 180m - 610m. The valleys are important farming areas, yet are subject to gully and sheet erosion.
The climate in the foothills is similar to that of the coastal plains; the mean annual temperature is 16C., with a mean minimum temperature in January of 3C. The frost-free season is 220 - 340 days, and again, the winter growing season has only light frost. Rainfall is heaviest from November to April and ranges varying from 0.3 - 0.5m per year depending on elevation and location. The parent material of the soils in this area and east to the mountains is decomposed granite, containing quartz fragments and other minerals. When carried by storm runoff, these materials act as an abrasive. The soils are generally classified as sandy loams and vary widely in depth. Organic matter is oxidized during the long, dry summer; thus, the soils are low in organic-carbon content.
The topography of the mountain zone is rugged, with steep-walled bouldery peaks. The elevation ranges from approximately 600 to over 1800m. The mountain range has a northwest-southeast trend but is broken by faults and river valleys. The steep topography, the rockiness, and shallow depth of topsoils make the greater part of this area unusable for crop cultivation.
Vegetation is more abundant in the mountains than in other parts of the area and consists of various pines, Douglas fir, white fir, black oak, interior live oak, and incense-cedar. There are also areas of grassland and chaparral. The cool climate slows the rate at which micro-organisms reduce the supply of organic matter, so these soils have the highest organic-carbon content of any soils in the study area. Soils under the pine and oak trees have mats 2 -14cm thick of fresh and somewhat decomposed needles, leaves, and twigs. The mountain area is the wettest part of the area, receiving 30 - 100cm of precipitation, and the coolest, with an annual mean temperature of 13C. The mean minimum temperature in January is 0C and the frost-free season is 150 - 200 days.
At the edge of the study area and extending to the east is the desert, which includes areas of Borrego badlands, lacustrine deposits, and very rocky barren hills. It is an area of nearly level to moderately sloping alluvial fans and plains. Since the desert lies in the rain shadow of the mountains, it receives the least precipitation and has the least vegetation of any region in the survey area. Total annual rainfall ranges from less than 13cm - 25cm, and for long periods there may be very little precipitation. Even when it does rain, however, there is not enough plant cover to control erosion. The desert has a wide range of seasonal and daily temperatures. The mean annual temperature is 22C, and the mean minimum temperature in January is 2C. The high temperature increases the rate of oxidation, so the organic-carbon content of the soils is very low. The frost-free season is 240 - 270 days. Soils in the desert area of the study tend to be saline-alkaline, because most of the moisture evaporates, leaving dissolved salts.
While not recognized as a physiographic province in the Natural Resource Conservation Service soils reports, the Temecula Valley, which includes the towns of Temecula, Elsinore, and Hemet, is perhaps the most significant physical subregion in the study area. Located north-east of MCB Camp Pendleton, this area is experiencing rapid urbanization. The terrain, climate, and soils combine to make the Temecula Valley relatively easy to convert to urban uses. The climate of the valley is Mediterranean, hot and dry in summer and cool and moist in winter. From May to October the rainfall averages 1.8cm at the lower elevations. Precipitation increases from September to November, peaks in December and January, and decreases rapidly in March and April. The soils in the area are typically well drained and deep, with a surface layer of sand or sandy loam. Vegetation is made up mostly of annual grassland, brushy chaparral, and sage. Some areas are devoted to orchard, vineyard, and other irrigated agricultural uses. Most of the valley is in the Santa Margarita River basin, which flows through Camp Pendleton. This is the subregion within which most of the future development associated with forecasted population growth is expected to occur.