Impact Assessment Study of Climate Change on
Evapotranspiration and Irrigated Agriculture
in the San Luis Valley, Colorado
Finnerty, B., and J. A. Ramirez, 1995: ‘Impact Assessment Study of Climate Change on Evapotranspiration and Irrigated Agriculture in the San Luis Valley, Colorado’, AWRA 31st Annual Conference and Symposia, Houston, TX Nov.
Office of Hydrology
NOAA/National Weather Service
1325 East-West Highway
Silver Spring, Maryland 20910
Jorge A. Ramirez, Ph.D.
Hydrologic Science and Engineering Program
Deptartment of Civil Engineering
Colorado State University
Fort Collins, Colorado
ABSTRACT: The impacts of CO2, temperature, precipitation, and
table variations on evapotranspiration (ET) and irrigated
were assessed. The sensitivity of ET to both CO2 and air
changes was evaluated using the modified Penman-Monteith
This equation accounts for the effects of atmospheric CO2 on
stomatal resistance, as well as air temperature changes on
land-surface-atmosphere water vapor exchanges. A root zone soil
balance was performed using a real-time, physically-based
soil-crop-climate model to analyze the sensitivity of soil
moisture to these
climate-induced changes. The economic sensitivity of potato
production to potential changes in available irrigation water and
agro-economic parameters was analyzed and compared with the
climatic impacts on agriculture.
The San Luis Valley is of great importance to Colorado's agricultural economy
and contains vast water resources which are of interest to agricultural, urban
and down stream water users. Anthropogenic impacts on land-surface features, emissions
into the atmosphere of greenhouse gases such as carbon dioxide [CO2], and natural
climate variability have a significant effect on water mass and energy budgets,
thus affecting hydrologic system response, weather, and climate (Cotton and Pielke,
Atmospheric CO2 levels are expected to double pre-industrial
revolution levels by some point in the next century, and are
to have many potential impacts on climate, vegetation, and
General Circulation Model (GCM) simulations under 2xCO2 scenarios
simulate a global climate with temperature increases from 2 C to
C, with regional temperature changes from -3 C to +10 C.
Precipitation is expected to vary in the range of +20% to -20%
current regional averages (Peterson and Keller, 1990).
depth to the water table may result from climate induced changes
groundwater recharge or from human consumption. Increased
CO2 is also known to effect many plant species' stomata, which
transpiration, as well as increase plant biomass production by
enhancing photosynthesis (Morison, 1987; Rosenberg, 1981, 1988).
To more effectively manage the water resource of the San Luis
Valley in a changing climate this work assessed the possible
of many different climate scenarios. Given the uncertainty in
predicting climate changes and climatic variability, a wide range
potentially plausible climate change scenarios were analyzed:
3 C increase and decrease in air temperature; 50% and 100%
in atmospheric CO2 concentrations; both a 25% increase and
precipitation volumes; and water table depths ranging from 1 to 3
meters below the soil surface. The fertilization effects of CO2
crop production was also analyzed by assuming a 50% increase in
biomass production for the 2xCO2 scenario.
Evapotranspiration (ET) is driven and controlled by the
climatic conditions that exist at the land surface-atmosphere
boundary, and the physiological characteristics of the
Potential evapotranspiration (PET) is the upper bound on actual
evapotranspiration (AET), and represents the vertical flux rate
exists if water supply in the soil-plant system is not
PET is controlled by atmospheric demands while AET is controlled
available soil moisture. Aside from small experiments,
evapotranspiration can not be measured at large scales. However,
there are many ET estimation techniques available at various
and spatial scales.
The development of a physically and physiologically based
soil-plant-climate evapotranspiration model requires all energy,
transfer, stomatal resistance, and crop aerial resistance terms.
modified Penman-Monteith equation is such a model, capable of
assessing the impacts of CO2 and temperature climate change
on evapotranspiration (Monteith, 1965). Many GCMs use this
to assess energy and vapor exchanges at the Earth's surface,
well suited for analyzing the sensitivity of PET to climate
scenarios. The Penman-Monteith equation is considered to be
universally applicable because it is derived from the energy
conservation equations (Fennessey and Kirshen, 1994). Details
concerning the data requirements and application of the PET model
the course of the growing season were presented by Finnerty
Morison (1987) compiled the results of the effects of 2xCO2
experiments performed on 16 C3 species, and found stomatal
rs increased 67% over present values. The data indicated a
linear relationship between atmospheric CO2 concentrations (Ca)
and was assumed to apply to potatoes. This relationship was used
assess the impacts of increased CO2 on PET rates of potatoes
C3 type photosynthetic pathways. The results of this research
applicable toward other C3 species grown in the San Luis Valley.
Accounting for seasonal changes in crop roughness height,
stomatal resistance, and climate parameters, mean daily PET rates
each growth period (GP) of the potato crop were evaluated.
change scenarios for temperature, CO2, and combinations of the
analyzed. The modified Penman-Monteith equation estimates daily
rates using daily or monthly data with 5-15% accuracy of measured
field data (Van Bavel, 1966; Szeicz et al., 1967; Jensen et al.,
1971). Measured PET rates for potatoes in the San Luis Valley
from 4.5 to 7.7 mm/day, which is very consistent with the
PET estimates presented in Table 1 (Troolen, 1988). The results
presented in Table 1 were obtained by changing only those
of the Penman-Monteith equation which are a function of
a function of CO2. The combined climate scenarios were assumed
an independent and additive effect on PET, and neglect any
processes between temperature, stomatal resistance, and PET.
analysis neglected climate change scenarios related to changes in
solar radiation, relative humidity, and wind speed. These issues
not addressed because of the uncertainty associated with
changes to these variables in mountainous regions.
The results of the analysis presented in Table 1 indicated
the PET rates of a potato crop are very sensitive to changes in
climatic temperature, atmospheric CO2 concentrations, and
of the two. All singular climate change scenarios analyzed
PET, with the exception of a temperature increase. The most
interesting results were those related to the combined changes of
temperature and CO2. These illustrated that the CO2 climate
had a greater effect on PET than temperature changes, for the
considered, and neglecting possible feedback processes. An
in temperature does increase PET; however the combination of a 3
temperature increase with a doubling of CO2 resulted in a 18.5%
decrease in PET. In addition, a 3 C temperature decrease
with a doubling of CO2 showed a 39% decrease in PET (Ramirez and
In conclusion, the analysis demonstrated PET rates for
and other C3 vegetation species growing in the San Luis Valley
reduced in a CO2 rich atmosphere, regardless of potential
changes. The effects of increased CO2 on PET rates dominated
increased temperature for the climate change scenarios analyzed.
Table 1: Potential Evapotranspiration Rates, mm/day
|Temp +3.5 C
|Temp -3.5 C
|Temp +3 C, 2xCO2
|Temp -3 C, 2xCO2
SOIL MOISTURE DEPLETION
A water balance was performed to investigate the impacts of
climate driven PET and precipitation changes, water table
fluctuations, and CO2 fertilization effects on soil moisture
processes and agricultural benefits. These changes modified the
temporal evolution of soil moisture content throughout the
season and consequently had a large impact on optimal irrigation
decisions and agricultural benefits. Ramirez and Bras (1982,
presented the details of the physically-based, theoretical
soil-crop-climate model capable of incorporating changes in
rates, and depth to the water table into optimal irrigation
Derivation of the capillary rise model as a function of soil
content and depth to the water table was presented by Finnerty
Impacts of Temperature-CO2 Changes on Agriculture
The results displayed in Table 1 for the cases of combined
temperature-CO2 scenarios on PET were used to derive AET curves
function of time. Figure 1 shows how decreasing PET acted to
the ratio of AET/PET in time. The crops were evapotranspiring at
lower rates, but that rate was closer to their potential rate.
resulted in a slowing of the soil moisture depletion rate as
Figure 2. The ratio of AET/PET was a surrogate measure of crop
moisture stress in the crop model, and was used to evaluate
irrigation schedules required to obtain maximum expected
benefits. Figure 3 shows the increase in the AET/PET ratio
in lower crop stress, water conservation, and higher crop yields,
which translated into higher expected agricultural benefits.
also shows that increasing available irrigation water increased
moisture through irrigation applications, which resulted in
crop moisture stress and increased agricultural benefits.
available irrigation water reduced the relative impact of
changes on agricultural production, while decreasing irrigation
increased the impact of climate-induced PET changes (Ramirez and
Figure 1. Combined Effects of Temperature and CO2 on %AET/PET
Figure 2. Combined Effects of Temperature and CO2 on Soil
Depletion Curves. Wmax=0.88 mm/day.
Figure 3. Combined Effects of Temperature, CO2, and Available
Agricultural Benefits. Wmax=0.88 mm/day.
The combined impacts of temperature and CO2 are very
when considering that temperature changes are uncertain as to
sign and magnitude, while CO2 is expected to double in the next
century. This analysis indicated that increasing atmospheric CO2
concentrations had a positive effect on irrigated agriculture,
regardless of potential temperature changes, for the climate
scenarios analyzed. Analysis of the single climate change
of temperature and CO2 can be found in Ramirez and Finnerty
Water Table Fluctuations
A shallow water table exists at the study site, ranging from
4 meters below the soil surface (Yenter et al., 1980). Water
variations may be caused by natural dynamics in groundwater
or from irrigation pumping schedules and other consumptive uses.
contribution to root zone soil moisture from a shallow water
increases as the depth to the water table decreases and is a
component in the soil water balance. Table 2 illustrates the
of water table fluctuations on the maximum rate of capillary
Wmax. The effect of capillary rise on soil moisture depletion
is illustrated in Figure 4. As the soil moisture content
the capillary potential in the soil increased, and consequently
capillary rise. The depletion curves exhibited asymptotic
converging in time to a soil moisture concentration where
rise was balanced by actual evapotranspiration. The same
found for AET functions, converging on the point where AET of
out of the soil was constant and equal to the contribution of
capillary rise into the soil (Ramirez and Finnerty, 1995b).
Increases in AET attributed to decreasing the depth to the
table reduced crop water stress and increased crop yields.
rise was most important to agriculture when there is limited
irrigation water because capillary rise substitutes for
water requirements and is provided at no cost to the farmers. As
available irrigation water increased, capillary rise became less
significant because irrigation water satisfied the plant water
Table 2: Capillary Rise with Depth to Water Table
Figure 4. Effects of Depth to the Water Table Z on Capillary Rise
and Soil Moisture Depletion.
The high variability of the spatial distribution of global
precipitation causes a large degree of uncertainty concerning
and local precipitation changes resulting from various
CO2-induced climate change scenarios. There is also a lack of
understanding of potential changes in rainfall characteristics
storm intensity, duration, and arrival rates. The impact
of precipitation changes on agriculture analyzed the following
scenarios: both a 25% increase and decrease in storm intensity,
duration, and storm arrival rates. For the analysis
characteristic changes were made independently, while holding all
other processes constant. The precipitation changes were applied
the entire growing season for all homogeneous precipitation
and were shown to impact soil moisture dynamics and agricultural
The results obtained from the analysis showed agriculture to
relatively insensitive to climatic precipitation changes for the
analyzed (Ramirez and Finnerty, 1995b). Precipitation provides a
minor portion of crop water use requirements in the San Luis
irrigated agricultural production. The valley's low
quantity of 76 mm/acre/season is small as compared to the 381
mm/acre/season of irrigation water used. Varying precipitation
(19 mm/acre/season) had little or no effect on irrigation water
requirements and agricultural benefits.
CO2 FERTILIZATION IMPACTS
Doubling of atmospheric CO2 has been experimentally proven to
increase crop yield and biomass production by 43% to 75% in
crops (Collins, 1976). Root and tuber crops were found to
marketable yield by an average of 52%, as observed in 17
of doubled atmospheric CO2 concentrations (Kimball and Idso,
This result shows that increasing CO2 will significantly increase
maximum crop yields and expected agricultural benefits given the
production and irrigation costs that currently exist.
Actual evapotranspiration was reduced even when biomass and
leaf area were increased due to increased atmospheric CO2
concentrations (Morison and Gifford, 1984; Idso et al., 1986).
However, there is uncertainty concerning the effect of increased
biomass on PET rates for CO2 fertilized plants. Because of this
uncertainty three cases were analyzed to investigate the issues
fertilization and crop water use efficiency. Case 1 is a 50%
in crop yield, combined with the historical PET rate. Case 2 is
increase in yield, coupled with the reduced 1.5xCO2 PET rate.
third case is a 50% increase in yield, coupled with the reduced
PET rate. The 1992 maximum potato yield was equal to 375
the 50% increase in maximum yield was 562.5 100lbs/acre (Colorado
Agricultural Statistics, 1992).
Table 3 shows CO2 fertilization had a very large impact on
irrigated agricultural benefits. The crop yield increase of 50%
combined with a 15% to 29% reduction in PET (see Table 1),
significant increases in agricultural benefits for all cases of
available irrigation water. The results illustrate how
benefits almost doubled under doubled CO2 concentrations, due to
large increase in crop yield coupled with increased crop water
efficiency. CO2 fertilization could make agriculture
feasible in regions with high production and irrigation costs,
where available irrigation water is constrained. In addition,
fertilization had a much greater positive effect on agricultural
benefits than the effects of CO2 on stomatal resistance and PET
Table 3: CO2 Fertilization Effects on Agricultural Benefits
|AVAILABLE IRR. WATER
ECONOMIC SENSITIVITY OF AGRICULTURE
The laws of supply and demand establish crop market values,
influence production decisions concerning acres to be planted,
irrigation water requirements, and production costs. These
are made prior to the start of the growing season when a large
of uncertainty exists. Farmers are at financial risk due to the
uncertainty associated with future crop market prices, natural
variability, unforeseen production costs, and natural disasters.
field of agro-economics is very complex and often site specific.
Therefore, this analysis only addresses the main economic issues
related to potato production in Conejos County, in the San Luis
Production costs are difficult to predict at the beginning of
season because of unforeseen production problems. The primary
objective of agriculture is to minimize production costs while
maximizing crop yields, so as to maximize financial benefits.
two objectives are in direct opposition because increasing
production generally increases crop yield. This makes it
evaluate the marginal value of money invested in crop production.
Given no information concerning future production costs, only
short-term climate variability in temperature and precipitation
analyzed. These climate changes were made while assuming current
atmospheric CO2 concentrations. The results of the analysis
maximum expected benefits for all climate change scenarios to be
greater than the production costs, given sufficient available
(400 mm/acre/season) and a significant contribution of capillary
(0.88 mm/day). However, a 10% increase in production costs
all profits for all temperature and precipitation change
analyzed. Conversely, a decrease in production costs, or an
in production efficiency would increase profit margins (Ramierz
Finnerty, 1995b). A 10% increase in production costs had a
impact on agriculture than +/-14% changes in PET or +/-25% change in
Market Value of Crops
Farmers generally plan for an average crop market value at
harvest time. However, the price of Colorado potatoes has
around the ten year mean of $4.65/100lbs, from a fifteen year low
$2.10/100lbs in 1987, to a high of $8.10/100lbs in 1989 (Colorado
Agricultural Statistics, 1992). This extreme annual variation in
market value makes it difficult for farmers to plan a production
strategy, especially when high production years may result in low
market values because of the excess supply at harvest time.
The results of the analysis on the impacts of crop market
variations on agricultural benefits are displayed in Table 4.
table shows a 50% increase in crop market price caused a very
significant increase in agricultural benefits, while a 50%
market value resulted in a devastating reduction of financial
benefits, causing the industry to loose money regardless of ample
water supply and high productivity (Ramirez and Finnerty, 1995b).
Table 4: Impact of Crop Market Value on Benefits, $/acre
SUMMARY AND CONCLUSIONS
1. All temperature and CO2 climate change scenarios had a
impact on evapotranspiration, soil moisture depletion, and
agriculture, with the exception of a temperature increase alone.
2. Long-term expected changes in CO2 had a larger impact than
temperature changes, for the scenarios analyzed.
3. CO2 fertilization effects had a significantly larger positive
impact on agricultural production than any of the other climate
induced changes to agricultural benefits.
4. Small variations in the depth to the water table
impacted the contribution of capillary rise to root zone soil
and agricultural benefits.
5. Agricultural sensitivity to agro-economic parameters had a
impact on agricultural benefits than any of the climate change
6. Irrigated agriculture in the San Luis Valley was essentially
insensitive to plausible precipitation changes.
7. Available irrigation water was crucial to the irrigated
agricultural economy of the San Luis Valley, Colorado. The crop
use requirements had to be met either from precipitation,
rise, or irrigation. Irrigation was capable of supplementing any
reduction of soil moisture caused by increased PET, lowering of
water table, or decreased precipitation. However, those
water resources needed to be available to reduce agricultural
attributed to damaging climatic or economic conditions if
production was to remain profitable in the region.
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