2.2 MEDITERRANEAN-TYPE CLIMATE 



Cool, wet winters and warm, dry summers 

 characterize the climate of southern 

 California. Most days are sunny, although 

 morning fog is common along the coast. Solar 

 radiation data for Chula Vista (the station 

 nearest Tijuana Estuary) were collected in 

 1976 and 1977; the average was 411 

 calories/cm 2 /day, ranging from an average of 

 561 for June to 217 for December (Taylor 

 1978). Griner and Pryde (1976) state that 

 San Diegans enjoy about 73% of the maximum 

 possible sunshine. Daily temperatures 

 average 17°C. (63°F), with monthly highs in 

 July or August (26°C, 78°F) and lows in 

 January (7°C, 45°F). Frost is rare along the 

 coast. 



The coastal climate of southern California 

 is reminiscent of the Mediterranean region 

 (Barbour and Major 1977), and the coastal 

 vegetation is similar to that of France, 

 southern Africa, and southwestern Australia, 

 all of which have Mediterranean-type 

 climates. For coastal wetland vegetation, 

 average rainfall on the coast is unlikely to be 

 the direct determining factor; rather, the 

 timing and amounts of rainfall and river flows 

 within the entire watershed set limits on 

 species distribution and growth. For 

 intertidal organisms, conditions that cause 

 high evaporation rates are also limiting. Hot, 

 dry desert winds can be devastating, 

 especially when they coincide with low tide. It 

 is a climate with many extremes; some years 

 and decades have little rainfall and runoff; 

 other years have winter storms that cause 

 tremendous floods. However, precipitation 

 data indicate that storms are always of brief 

 duration. 



Data for six stations within the Tijuana 

 Estuary watershed (Figure 2.4, Table 2.1) 

 show the seasonal patterns of rainfall and 

 evaporation. At all stations, evaporation 

 exceeds precipitation in nearly every month. 

 The more inland stations have lower ratios of 

 evaporation and precipitation, as is typical of 

 the western sides of mountain ranges (Table 

 2.1). Annual rainfall averages are about 25 



Table 2.1. Locations of weather stations 

 within the Tijuana River watershed (see 

 Figure 2.4). 



Station 



Lat. 



Long. 



Elev. 



UNITED STATES 

 Morena Dam 

 Barrett Dam 

 Lower Otay Dam 



32°41' ,.116°31' 938 

 32°41' 116°40' 495 

 32°37' 116°56" 156 



MEXICO 



San Juan de Dios 32°59' 1 1 6°00' 



1280 



ValleRedondo 32°31" 116°45" 24 2 



cm (10 inches), most of which falls between 

 November and April. Although some winter 

 months have heavy rainfall, monthly averages 

 are less than 5 cm (2 inches). Annual 

 evaporation rates are very high. Averages at 

 Chula Vista for 1919-1981 (IBWC 1981) 

 are 161 cm (64 inches) per year, with the 

 maximum occurring in July (19 cm = 7.6 

 inches per month). Even in winter, 

 substantial water losses can occur. The 

 minimum monthly average evaporation is for 

 December, with 7 cm (2.8 inches). 



Although rainfall patterns influence the 

 intertidal portions of the estuary, 

 temperature and salinity data for channel 

 waters show that the subtidal habitats are 

 much less variable (Table 2.2). When 

 Tijuana Estuary is open to tidal flushing, 

 monthly water temperatures vary only 

 slightly, and water salinities change 

 primarily with major river flows. 



The rainfall data for San Diego extend back 

 to 1880 and indicate that periods of relative 

 drought were interrupted by wet years in 

 1883, 1921, 1940, 1951, 1978, 1980, 



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