Watersheds
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Dan Neary
Southwest Watershed Science Team
Rocky Mountain Research Station
2500 S. Pine Knoll Lane
Flagstaff, AZ 86001

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Managing Arid and Semi-Arid Watersheds

Climate

Climate is essentially the condition of the weather resulting from the interactions between air temperature, precipitation, and land form. Climate has a major influence over plant distribution (Figure 1) and the characteristic of streamflow generation in the area. The climate of the southwestern United States is a cycle of winter precipitation, spring drought, summer precipitation, and fall drought. Winter precipitation—moisture received between October and April—usually comes from a northwesterly directions from the Pacific Ocean. This precipitation is associated with frontal or large regional storms falling as snow at elevations above 6,000 feet and mainly as rain below the 3,000 foot level. Winter precipitation is responsible for 80 to 95% of the annual streamflow produced in the region.

Figure 1: Elevational - Precipitation Relationships of Biotic Communities
Figure 1

Riparian Ecosystems

Climatic characteristics of riparian ecosystems exhibit a wide range of conditions due to large elevational differences and distributions of associated mountain ranges and highlands. The key characteristic of the riparian system is the availability of water throughout the year or at least during the growing season.

Riparian ecosystems occur in elevations from near sea level up to 10,000 feet and occupy about 0.1 million acres in Arizona. Water yield averages from 6 to 24 inches depending on precipitation, elevation, soils, and vegetation density.

Riparian areas provide habitat for a numerous variety of wildlife species, grazing for livestock and wildlife, and areas for fishing, hiking, bird watching, picnicking, and camping.


High elevation riparian area

Low elevation riparian area

Mixed Conifer Forests

Mixed conifer forests grow between elevations of 7,000 to 10,000 ft and occupy about 0.4 million acres in Arizona. Annual precipitation in the high elevation mixed conifer forests (above 9,500 feet) ranges from 30 to 45 inches and is normally in excess of potential evapotranspiration—the amount of water required by plants to grow normally. As a result of this excess precipitation, streams originating in this area are often perennial—flow year round. Streams originating in the lower elevation mixed conifer forests (8,000 to 9,500 feet) are mostly intermittent. Snowmelt runoff is the primary source of annual runoff.

Water yield from natural forest stands range from 3 to 5 inches depending on precipitation, elevation, soils, and overstory density. Potential increases in water yield of 3 to 4 inches have been obtained by manipulating the forest overstory with 1.5 inches expected if one maintains one-third of the watershed in openings for site with 4 to 5 inches of normal water yield. See Mixed Conifer Treatments and Results for more information.

Grasslands

  • Mountain grasslands ranges from 30 to 45 inches of annual precipitation, with 50% occurring during the summer season. The general weather patterns of these areas coincide with those of the adjacent forests.


Mountain Grassland site

  • Plains grasslands average about 17 inches of average annual precipitation, with extremes of approximately 10 and 20 inches. About 70% of the total annual precipitation falls between April and September.
  • Desert grasslands—The climate of the desert grasslands is generally characterized by warm to hot summers, and by mild and open winters. This grassland type is the most arid of all North American grassland regions.

Plains Grassland site

Desert Grassland site

Precipitation varies from 1 to 12 inches annually, averaging about 6 inches. Most of the precipitation occurs as liquied rainfall during two seasons, summer and winter. Summer rains are largely in the form of thunderstorms, with high-intensity events, with rainfall occurring intermittently for several days. Normally, relative humidities are low throughout the year, except during the storm periods.

Temperatures and wind velocities are often high in the desert grasslands, and evaporation rates are high. The high temperature and wind velocities, coupled with the low humidity, account for the comparatively high evaporation rates.

Ponderosa Pine Forests

The ponderosa pine forest type is found between elevations of 6,000 and 9,000 feet on the Mogollon Plateau and on scattered "sky island" areas in the Southwest and occupy nearly 6 million acres in Arizona. High rates of water use by plants and low soil moisture can often curtail the growth of plants in the ponderosa pine forests, which receive 20 to 30 inches of annual precipitation. Normally, little summer rain is converted into streamflow. Winter precipitation is the major source of runoff.

Water yield from natural forest stands range from 2 to 6 inches depending on precipitation, elevation, soils, and overstory density. As you go higher in elevation, you generally get more precipitation, and conditions are more favorable for less evapotranspiration loss. Potential increases in water yield of 0.1 to 1.0 inch can be expected by manipulating the forest overstory (1 to 3 inches in the short term, 3 to 10 years, have been measured (Baker, 1986)). See Ponderosa Pine Treatments and Results for more information.

Ponderosa pine forest in winter

Pinyon-Juniper Woodlands

The coniferous woodlands or pinyon-juniper types generally lie below the ponderosa pine forests, at elevations of 4,500 to 7,500 feet and occupies 19.9 millon acres in Arizona. Again, there are wide fluctuations in weather patterns throughout the pinyon-juniper woodlands. Annual precipitation varies from 12 to 24 inches. Winter precipitation is usually rain with occasional snow. Evapotranspiration (evaporation from soil, leaves, and needles) rates are relatively high in the growing season. Only during the coldest months of December through February is precipitation greater than the potential evapotranspiration rates. Streamflow is typically ephemeral—flowing only in the winter season during periods of precipitation excess.

Water yield from natural stands ranges from 1 to 3 inches depending on precipitation, elevation, soils, and overstory density. Potential increases of 0.5 inch or less can be expected from the pinyon-juniper woodlands. See Pinyon-Juniper Woodlands Treatments and Results for more information.

Thermal heating during winter storms is less pronounced than typically occurs during summer storm events. Upslope air movement is relatively slow, cloudiness is common, and precipitation tends to fall over a large area and at relatively low rainfall rates. These conditions have major implications on energy available to produce soil erosion and movement of sediment.

Chaparral Shrublands

Chaparral shrublands occupy 3.5 million ac in Arizona. Mean annual precipitation ranges from 16 inches at the lower limits of the chaparral (3,000 ft) to over 25 inches on the wetter sites (6,000 ft). About a third of the winter precipitation falls as snow. Summer storms are often intense, but produce less streamflow than the larger, less intense winter storms, which yield about 90% of the annual streamflow. Flow is intermittent under natural conditions.

Water yield ranges from 1 to 4 inches depending on precipitation, elevation, soils, and overstory density. Potential increase from less than 1 up to 5 inches have been measure as the result of vegetation conversion or overstory density reduction. See Chaparral Shrublands Treatments and Results for more information.


Lower Density Chaparral site

Higher Density Chaparral site

Desert Shrublands

The diversity of vegetative cover found within the desert shrub cover type would seem to make it difficult to characterize the climatic features of the area. Temperature is not necessarily a determining factor in the production of vegetation on a desert. High temperatures may accelerate the loss of water, as, conversely, continued temperatures below the physiological activity of the plants may produce drought conditions for the plants. Mositure, is the determining factor of production, with its more significant attribute being the distribution of the annual supply.

Average precipitation in the northern desert shrub type is about 10 inches annually, with a general variation of 5 to 14 inches. Annual precipitation in the southern desert shrub type varies from 3 to 12 inches, but averages approximately 6 inches.

The climatic conditions associated with the desert shrub type are extreme: high temperatures are combined with relatively low annual precipitation amounts. Additional physiological stress is placed upon the vegetation by the fact that the moisture supply is not evenly distributed throughout the growing season.


Desert Shrubland site

Desert Shrubland site

Winter Precipitation

Although winter precipitation accounts for only 50 to 60% of the annual precipitation in the southwestern United States, it is responsible for 80 to 95% of the annual streamflow produced in the region.  Winter precipitation—moisture received between October and April—usually comes from a northwesterly directions from the Pacific Ocean. This precipitation is associated with frontal or large regional storms falling as snow at elevations above 6,000 feet and mainly as rain below the 3,000 foot level.

Because winter precipitation normally occurs as low intensity rain or snow fall and streamflow is the product of either low intensity rainfall or snowmelt, erosion potential is energy limited and the dominant parent materials often limits the supply of sediment (e.g., heavy clay soil characteristic of volcanic-derived soil) or the transport of the dominant sediment size particles unless runoff is substantially concentrated (e.g., coarse textured soils derived from sedimentary and granitic parent materials).

Summer Precipitation

The major source of moisture for summer storms is the Gulf of Mexico. This moisture moves into Arizona from the southeast, passes over highly heated and mountainous terrain, and thermal heating causes it to rise rapidly, cool, and condense. Summer storms, therefore, are primarily convectional, often intense, and usually local or small in size (a square mile or less) rather than being widespread, regional storms. This characteristic influences the amount and distribution of energy available for producing erosion. Summer rains typically begin in early July, breaking the prolonged spring drought and providing relief from the hot weather of June and July.

Precipitation Variation

Annual—Variation in annual precipitation is probably the most important characteristic governing the climate of the Southwest. Winter precipitation is more variable than summer precipitation in both amount and time of occurrence from year-to-year. However, yearly variations in precipitation are generally less at higher elevations than at the lower elevations.

Seasonal—Spring drought—May and June—is often more severe than the fall drought—October-on most plants and animals in the region. This is due largely to the higher air temperatures and the higher activity levels of the plants and animals in the spring.