Local hydroclimate

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The mechanisms responsible for local hydroclimates are essentially the same as those creating global and regional hydroclimates. The major difference is local hydroclimate emphasizes the spatial variability of the surface characteristics and how they interact with the overlying atmosphere. The surface characteristics include the type of surface, the nature and size of objects on the surface, the general topography of the area, and altitude. Slope angle and aspect and local winds are important contributors to hydroclimate differences at this scale. Orographic winds, land and sea breezes, mountain-valley circulations, and winds associated with tropical cyclones influence the humidity and temperature characteristics of local hydroclimates, and the influence of these differences can be significant over relatively short distances (Linacre, 1992). The main physical consequence of these characteristics is that they produce variations in the radiation and energy balances and spatial temperature variations. In addition, surface characteristics produce local cloud and precipitation regimes that influence the water balance that is a fundamental expression of hydroclimatology.

Local hydroclimate produces the greatest diversity because it is a representation for a specific site. Another site only a few meters away may be very different due to variations in the site-specific characteristics controlling energy and moisture loadings. The areal range of local hydroclimates is typically less than a few km2, and the time step for expressing these characteristics is minutes or hours (Linacre, 1992). Because measurements of hydroclimate variables are site specific, a complementary relationship exists between local hydroclimate and regional and global hydro-climates. The perception of global and regional hydroclimates is founded on local hydroclimate measurements, and local hydroclimate results from the same mechanisms as those creating global and regional hydroclimates.

7.14.1 Local solar radiation

Solar radiation reaches the Earth's surface by both direct and diffuse pathways. The optical air mass and the amount of water vapor, cloud, and aerosols in the atmosphere all contribute to reducing the solar radiation arriving at the surface to drive hydroclimatic processes. Variations in atmospheric conditions produce differences in hourly solar radiation over short distances that are not apparent when values are averaged for longer time intervals (Linacre, 1992). Local variations in solar radiation establish the foundation for complex differences in hydroclimatic processes not evident at larger time and space scales.

Hourly solar radiation data for three sites in California's Sacramento Valley (Fig. 7.31) illustrate solar radiation differences over relatively short distances.

Hour

Fig. 7.31. Solar radiation on 7 May 2006 at three California Irrigation Management Information System (CIMIS) sites less than 20 km apart in California's Sacramento Valley. (Data courtesy of the California Department of Water Resources from their website at http://wwwcimis.water.ca.gov/.)

Hour

Fig. 7.31. Solar radiation on 7 May 2006 at three California Irrigation Management Information System (CIMIS) sites less than 20 km apart in California's Sacramento Valley. (Data courtesy of the California Department of Water Resources from their website at http://wwwcimis.water.ca.gov/.)

Table 7.4. Daily averages for selected climatic variables on 7 May 2006for three California Irrigation Management Information System (CIMIS) sites in California's Sacramento Valley

Variable

Davis

Dixon

Winters

Solar radiation (W m-2)

297

279

353

Vapor pressure (kPa)

1.3

1.5

1.4

Air temperature (°C)

19.1

18.1

20.1

Dew point temperature (°C)

11.0

12.5

12.2

Wind speed (m s-1)

1.6

2.6

1.5

Wind direction (0-360)

221

236

145

Fig. 7.32. The California Irrigation Management Information System (CIMIS) instruments at Davis, California. (Photo by author.)

The sites are part of the California Irrigation Management Information System (CIMIS) Network. The distances between the sites are 13, 16, and 18 km. Davis and Dixon are the closest, and Dixon and Winters are the most distant. Total incoming solar radiation is measured at each site with a pyranometer mounted 2 m above a grass-covered and unobstructed surface (Fig. 7.32). The average solar radiation for 7 May 2006 is shown in Table 7.4 along with averages for other selected variables for this date. The largest difference in average solar radiation is 27% for Dixon and Winters. The maximum hourly values range between 802 Wm~2 at Dixon and 1036 Wm-2 at Winters, or a 29% difference.

Earth-Sun geometry indicates solar radiation should be similar at the three sites. Observed differences are the product of the interplay of environmental influences acting to attenuate solar radiation. The air at Dixon is cooler and more humid, winds are stronger, and the dominant flow is from the west, southwest. This trajectory promotes the inland flow of marine air from the Pacific Ocean. These conditions are especially strong during the mid-day hours when the solar radiation differences between Dixon and the other sites are greatest. The elevated moisture content of air at Dixon provides greater opportunities for reducing incoming solar radiation below the levels observed at the other sites.

At Winters, winds prior to 1500 are from the north, northeast, the air is drier than at Dixon, and solar radiation is greater each hour as less is attenuated. Davis is the furthest north and east of the three sites, and the air at Davis is the driest. Light winds are variable from the south to north, northwest throughout the day. Between 1200 and 1400, winds are stronger at Davis than at the other sites and are from the north, northwest. Winds from this trajectory often have increased aerosols after flowing across agricultural lands, and these aerosols reduce solar radiation. This influence would account for the reduced mid-day solar radiation values at Davis, but Winters does not experience a similar influence because winds during these hours are weak and from the north and do not provide similar aerosol loadings.

7.14.2 Local precipitation

Precipitation, evapotranspiration, and soil moisture readily illustrate the character of local hydroclimatic variations when portrayed as point data. Point values avoid the issue of interpolation arising from the uncertain spatial characteristics of variables. Precipitation is measured with high space and time frequency and is recommended as an observed value for comparison at the local scale.

800 1100 1400 1700 2000 2300 200 500 800 Hour

Fig. 7.33. Precipitation on 5-6 March 2006 at three California Irrigation Management Information System (CIMIS) sites less than 20 km apart in California's Sacramento Valley. Solid column is Davis, California, shaded column is Dixon, California, and clear column is Winters, California. (Data courtesy of the California Department of Water Resources from their website at http://wwwcimis.water.ca.gov/.)

800 1100 1400 1700 2000 2300 200 500 800 Hour

Fig. 7.33. Precipitation on 5-6 March 2006 at three California Irrigation Management Information System (CIMIS) sites less than 20 km apart in California's Sacramento Valley. Solid column is Davis, California, shaded column is Dixon, California, and clear column is Winters, California. (Data courtesy of the California Department of Water Resources from their website at http://wwwcimis.water.ca.gov/.)

Hourly precipitation for 5-6 March 2006 at the three sites described in Section 7.14.1 is shown in Figure 7.33. The Dixon total is 37 mm and the total for both Davis and Winters is 38 mm. All three sites record the major precipitation occurrence during the 15 hours from 1500 on 5 March to 0500 on 6 March. The temporal rainfall occurrence is similar at Davis and Dixon with an initial pulse peaking at 2000, a maximum at 0200, and a comparatively rapid decline after the 0200 maximum. The hourly rainfall occurrence at Winters is distinctly earlier in the evening and contrasts markedly with the pattern at the other two sites. Precipitation reaches a maximum at Winters at 1800, declines through 2200, increases to a secondary peak at 0100, and generally declines thereafter. While Davis and Dixon record 24 mm and 25 mm, respectively, of their total between 2200 and 0600, Winters accumulates 23 mm of its total from 1400 to 2100. Winters has four hours that receive 3 mm or more of rainfall, while Davis and Dixon have three hours each. Winters maximum rainfall is 4.5 mm at 1800, and the maximum at Davis and Dixon of 6.1 mm and 5.6 mm, respectively, is at 0200. In summary, although the rainfall event totals are similar at the three sites, the temporal characteristics illustrate that important differences with hydroclimatic significance occur over small areas.

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