Regional hydroclimate

The Earth's surface can be divided into regions that have similar hydroclimates due to the global climate system, but regional hydroclimate variations elucidate the role of a second tier of factors responsible for the spatial variation in hydroclimate when viewed at a higher resolution than global patterns. Regional hydroclimates emerge as identifiable entities based on latitude, altitude, and orientation of the surface in relation to water bodies, mountains, and prevailing winds (Hartmann, 1994). Regional hydro-climates resulting from these second-tier factors are masked by the level of generalization used to characterize global hydroclimates. However, regional-scale hydroclimates are components of the global hydroclimatic system, and they form the mosaic pattern that gives detail to the expression of hydro-climate most apparent for human observers at the Earth's surface. Large river basins are often employed as a basis for quantifying hydroclimatic variables of interest. The precise dimensions of a hydroclimatic region are variable, but they commonly involve areas of several hundred to thousands of km2, and the time scales used for analysis are weeks, months, and years (Linacre, 1992).

7.13.1 Atmospheric hydroclimatic variables

Temperature serves to illustrate regional characteristics of atmospheric hydroclimatic variables due to the convenient availability of appropriate data for this variable. The temperature examples depict variations commonly observed at this scale for this class of hydroclimatic variables. Regional temperature characteristics are revealed by graphic portrayal of individual station data and by maps of monthly temperature.

A station's annual temperature regime is the sequence of mean monthly temperatures resulting from the multiple factors influencing temperature at a specific location. The three stations shown in Figure 7.23 span 5° of latitude along the Texas/Mexico border. The seasonal temperature variation of 23 °C at El Paso, Texas, is characteristic of the temperature resulting from the rapid heating and

Ojinaga,

J FMAMJ JASOND Month

Fig. 7.23. Monthly mean temperature for three stations along the Texas/Mexico border showing regional temperature variations. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php.)

Ojinaga,

J FMAMJ JASOND Month

Fig. 7.23. Monthly mean temperature for three stations along the Texas/Mexico border showing regional temperature variations. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php.)

cooling at inland locations. El Paso's elevation (1194 m) accounts for modest cooling of summer temperatures compared to those observed at Ojinaga, Mexico (elevation 841 m), 330 km southeast of El Paso. The January mean temperature is 5 °C warmer at Ojinaga which is 750 km from the Gulf of Mexico. Mean monthly temperatures at Rio Grande City, Texas, approximately 200 km from the Gulf of Mexico, display a seasonal temperature variation of 17 °C related to the moderating influence of the Gulf. Cool season temperatures are 3 °C warmer at Rio Grande City compared to Ojinaga and summer temperatures are 2 °C cooler at Rio Grande City. An additional indication of the temperature-moderating influence of the Gulf of Mexico is that August is the warmest month at Rio Grande City, but July is the warmest month at Ojinaga.

Maps of monthly temperature provide a spatial summary of regional temperature variations. National, state, or watershed boundaries are commonly employed to define the area examined, but any definable areal construct can be employed. State boundaries, such as California (Fig. 7.24), are a convenient delimiter. California covers 411000 km2 and extends 1280 km north to south. Mountains near the coastline range in maximum elevation from 2300 m in the north to 3600 m in the south, and mountains along the eastern border north of 35° N have

Fig. 7.24. Physical features of California.
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Fig. 7.25. California January mean temperature. Units in °C. (Data courtesy of the USDA Natural Resources Conservation Service and the Prism Group from their website at http://www.ncgc.nrcs.usda.gov/.)

peaks above 4400 m. California temperatures in January (Fig. 7.25) and July (Fig. 7.26) are increasingly warmer from north to south due to solar radiation being 15% greater at the southern border at 32° N compared to the northern border at 42° N (Peixoto and Oort, 1992). Temperature differences west to east across California are related to the presence of the Pacific Ocean that increases January temperatures and decreases July temperatures along the coastline compared to more inland areas. Elevation influences due to the north-south trending mountain ranges are especially evident in greater January cooling than seen elsewhere in the state. Slope and aspect influences combine with elevation to produce a more complex pattern of isotherms along the eastern border in July than in January.

7.13.2 Terrestrial hydroclimatic variables

Precipitation illustrates regional features of terrestrial hydroclimatic variables. Precipitation has a dominant role in the runoff process, and precipitation data are the most readily available of this class of variables. Precipitation data are readily graphed and mapped to depict regional-scale variations.

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Fig. 7.26. California July mean temperature. Units in °C. (Data courtesy of the USDA Natural Resources Conservation Service and the Prism Group from their website at http://www.ncgc.nrcs.usda.gov/.)

Graphic portrayal of the monthly precipitation sequence reveals valuable insights regarding the moisture supply for hydroclimatic processes. Mean monthly precipitation for four stations in southern Arizona and northern Mexico (Fig. 7.27) displays regional differences in the quantity and monthly occurrence of precipitation. These stations form a trapezoid approximately 350 km between the western apex at Ajo, Arizona, and the eastern apex at Douglas, Arizona. A summer-dominant precipitation pattern is evident at all four stations, but the annual precipitation varies from 377 mm at Douglas to 227 mm at Ajo. The July to September precipitation is attributed to the atmospheric circulation feature known regionally as the North American monsoon (Lorenz and Hartmann, 2006). This phenomenon is an expression of complex atmospheric circulation and water vapor transport that produces considerable temporal and spatial variability across a wide area of the southwestern United States and northwestern Mexico.

Month

Fig. 7.27. Monthly mean precipitation for four stations along the Arizona/Mexico border showing regional precipitation variations. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php, and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/ epubs/ndp/ushcn/usa_monthly.html.)

Month

Fig. 7.27. Monthly mean precipitation for four stations along the Arizona/Mexico border showing regional precipitation variations. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php, and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/ epubs/ndp/ushcn/usa_monthly.html.)

The average January (Fig. 7.28) and July (Fig. 7.29) precipitable water vapor over the United States portrays the large-scale atmospheric moisture field spatial characteristics. The general patterns evident in both months are decreasing values from south to north, decreasing values inland from the west coast and the south coast, and the effect of the Rocky Mountains in producing a relatively dry area centered on an axis at 110° W. These patterns result from temperature variations that influence the atmosphere's capacity to retain moisture, distance from oceanic moisture sources, and prevailing atmospheric motion that transports the moisture from oceanic sources.

The January (see Fig. 7.28) and July (see Fig. 7.29) contrast in mean precipi-table water across the Arizona/Mexico region reveals a significant seasonal disparity in atmospheric moisture. The magnitude of the seasonal water vapor variability is ultimately responsible for regional precipitation variability. July precipitable water is three times greater than January values across much of southern Arizona and northern Mexico. July precipitable water over the Arizona portion of the Mexico border region is second only to the Texas portion of the border, which benefits from its proximity to moisture inflow from the Gulf of Mexico resulting in elevated precipitable water values. The contrasting orientation of the precipitable water contours reveals another facet contributing to seasonal precipitation variability. The January contours across the Arizona/ Mexico region display a meridional pattern that magnifies east-west moisture variability across the region. The July contours have a zonal pattern that emphasizes north-south moisture differences across the region. Differences in

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Fig. 7.28. United States January mean precipitable water. Units in mm. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)
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Fig. 7.29. United States July mean precipitable water. Units in mm. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

precipitable water and surface influences contributing to instability help account for seasonal and spatial precipitation differences at individual sites.

The precipitation regime at Atil, Sonora (see Fig. 7.27), displays the strongest expression of the monsoon influence in that 53% of the annual precipitation of 351 mm arrives in July and August. Further east at Douglas, Arizona, July accounts for nearly 13% more precipitation than August, and the two months combine for 49% of the annual total. July and August precipitation at Tucson, Arizona, is about 60% of precipitation for these months at Atil, and they provide 40% of the 297 mm of annual precipitation at Tucson. On the west side of the region at Ajo, the monsoon influence accounts for a July and August contribution of 37% of annual precipitation. A notable feature of the monthly pattern at Ajo is that August precipitation is 61% greater than July precipitation. At the other three stations, July and August precipitation are relatively similar in magnitude.

Annual precipitation for California (Fig 7.30) displays regional variations related to its proximity to the Pacific Ocean, topography, and atmospheric circulation. Precipitation exceeding 200 cm near the northwest coast results

Fig. 7.30. California annual mean precipitation. Units in cm. (Data courtesy of the USDA Natural Resources Conservation Service and the Prism Group from their website at http://www.ncgc.nrcs.usda.gov/.)

Fig. 7.30. California annual mean precipitation. Units in cm. (Data courtesy of the USDA Natural Resources Conservation Service and the Prism Group from their website at http://www.ncgc.nrcs.usda.gov/.)

from the passage of cold fronts and cyclones imbedded in prevailing westerly winds encountering mountains perpendicular to the atmospheric flow. Much of the moisture in the air from the Pacific Ocean moving onshore is removed by orographic influences. Decreasing precipitation from north to south along the coastline results from seasonal shifts in storm tracks and a decreasing occurrence of rain-producing storms related to latitudinal changes in the position and strength of the Pacific subtropical high-pressure system (Castello and Shelton, 2004). Storm tracks move southward in the fall and winter as the westerlies increase in strength in response to the stronger Northern Hemisphere pressure gradient. Frontal tracks move poleward in the spring and summer as the Pacific subtropical high-pressure cell strengthens, moves poleward, and asserts domination of the general circulation in these latitudes. Annual precipitation on the southern California coast at San Diego is 24 cm.

Inland locations reflect similar north to south precipitation differences, but complexity is added by elevation, slope orientation, distance from the ocean, and the nature of intervening topography. Precipitation at lower elevations in central and southeastern California is 20 cm or less as these areas are downwind of the coastal mountains. Extreme dryness occurs in the southeastern desert where meager winter rainfall and summer thunderstorms produce only 6 cm of annual precipitation at Imperial near the Salton Sea. The high mountains in east-central and northern California produce large precipitation amounts as marine air is forced to ascend a second time in its west to east trajectory. Annual precipitation at Nevada City, west of Lake Tahoe, is 139 cm.

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