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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 water table variations on evapotranspiration (ET) and irrigated agriculture were assessed. The sensitivity of ET to both CO2 and air temperature changes was evaluated using the modified Penman-Monteith equation. This equation accounts for the effects of atmospheric CO2 on plants' stomatal resistance, as well as air temperature changes on land-surface-atmosphere water vapor exchanges. A root zone soil water 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 potential 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, 1992).

Atmospheric CO2 levels are expected to double pre-industrial revolution levels by some point in the next century, and are expected to have many potential impacts on climate, vegetation, and hydrology. General Circulation Model (GCM) simulations under 2xCO2 scenarios simulate a global climate with temperature increases from 2 C to 5 C, with regional temperature changes from -3 C to +10 C. Precipitation is expected to vary in the range of +20% to -20% from current regional averages (Peterson and Keller, 1990). Variations in depth to the water table may result from climate induced changes in groundwater recharge or from human consumption. Increased atmospheric CO2 is also known to effect many plant species' stomata, which control 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 impacts of many different climate scenarios. Given the uncertainty in predicting climate changes and climatic variability, a wide range of potentially plausible climate change scenarios were analyzed: both a 3 C increase and decrease in air temperature; 50% and 100% increase in atmospheric CO2 concentrations; both a 25% increase and decrease in precipitation volumes; and water table depths ranging from 1 to 3 meters below the soil surface. The fertilization effects of CO2 on 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 physical climatic conditions that exist at the land surface-atmosphere boundary, and the physiological characteristics of the vegetation. Potential evapotranspiration (PET) is the upper bound on actual evapotranspiration (AET), and represents the vertical flux rate that exists if water supply in the soil-plant system is not constrained. PET is controlled by atmospheric demands while AET is controlled by 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 temporal and spatial scales.

The development of a physically and physiologically based soil-plant-climate evapotranspiration model requires all energy, mass transfer, stomatal resistance, and crop aerial resistance terms. The modified Penman-Monteith equation is such a model, capable of assessing the impacts of CO2 and temperature climate change scenarios on evapotranspiration (Monteith, 1965). Many GCMs use this equation to assess energy and vapor exchanges at the Earth's surface, making it well suited for analyzing the sensitivity of PET to climate change 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 over the course of the growing season were presented by Finnerty (1994).

Morison (1987) compiled the results of the effects of 2xCO2 experiments performed on 16 C3 species, and found stomatal resistance rs increased 67% over present values. The data indicated a general linear relationship between atmospheric CO2 concentrations (Ca) and rs, and was assumed to apply to potatoes. This relationship was used to assess the impacts of increased CO2 on PET rates of potatoes which have C3 type photosynthetic pathways. The results of this research may be 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 for each growth period (GP) of the potato crop were evaluated. Climate change scenarios for temperature, CO2, and combinations of the two were analyzed. The modified Penman-Monteith equation estimates daily PET 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 range from 4.5 to 7.7 mm/day, which is very consistent with the historical PET estimates presented in Table 1 (Troolen, 1988). The results presented in Table 1 were obtained by changing only those parameters of the Penman-Monteith equation which are a function of temperature or a function of CO2. The combined climate scenarios were assumed to have an independent and additive effect on PET, and neglect any feedback processes between temperature, stomatal resistance, and PET. This analysis neglected climate change scenarios related to changes in solar radiation, relative humidity, and wind speed. These issues were not addressed because of the uncertainty associated with CO2-induced changes to these variables in mountainous regions.

The results of the analysis presented in Table 1 indicated that the PET rates of a potato crop are very sensitive to changes in climatic temperature, atmospheric CO2 concentrations, and combinations of the two. All singular climate change scenarios analyzed reduced 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 changes had a greater effect on PET than temperature changes, for the cases considered, and neglecting possible feedback processes. An increase in temperature does increase PET; however the combination of a 3 C temperature increase with a doubling of CO2 resulted in a 18.5% decrease in PET. In addition, a 3 C temperature decrease combined with a doubling of CO2 showed a 39% decrease in PET (Ramirez and Finnerty, 1995a).

In conclusion, the analysis demonstrated PET rates for potatoes and other C3 vegetation species growing in the San Luis Valley were reduced in a CO2 rich atmosphere, regardless of potential temperature changes. The effects of increased CO2 on PET rates dominated those of increased temperature for the climate change scenarios analyzed.

Table 1: Potential Evapotranspiration Rates, mm/day

HISTORICAL 4.47 6.70 7.24 5.84 0.0
Temp +3.5 C 5.10 7.52 8.08 6.51 +13.8%
Temp -3.5 C 3.89 5.91 6.43 5.19 -13.8%
1.5xCO2 3.70 5.67 6.18 4.97 -15.2%
2xCO2 3.16 4.92 5.40 4.33 -29.4%
Temp +3 C, 2xCO2 3.65 5.61 6.12 4.91 -18.5%
Temp -3 C, 2xCO2 2.71 4.28 4.72 3.79 -39.3%


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 depletion processes and agricultural benefits. These changes modified the temporal evolution of soil moisture content throughout the growing season and consequently had a large impact on optimal irrigation decisions and agricultural benefits. Ramirez and Bras (1982, 1985) presented the details of the physically-based, theoretical soil-crop-climate model capable of incorporating changes in precipitation, PET rates, and depth to the water table into optimal irrigation decisions. Derivation of the capillary rise model as a function of soil moisture content and depth to the water table was presented by Finnerty (1994).

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 as a function of time. Figure 1 shows how decreasing PET acted to increase the ratio of AET/PET in time. The crops were evapotranspiring at lower rates, but that rate was closer to their potential rate. This resulted in a slowing of the soil moisture depletion rate as shown in Figure 2. The ratio of AET/PET was a surrogate measure of crop moisture stress in the crop model, and was used to evaluate optimal irrigation schedules required to obtain maximum expected agricultural benefits. Figure 3 shows the increase in the AET/PET ratio resulted in lower crop stress, water conservation, and higher crop yields, which translated into higher expected agricultural benefits. Figure 3 also shows that increasing available irrigation water increased soil moisture through irrigation applications, which resulted in reduced crop moisture stress and increased agricultural benefits. Increasing available irrigation water reduced the relative impact of climatic PET changes on agricultural production, while decreasing irrigation water increased the impact of climate-induced PET changes (Ramirez and Finnerty, 1995a).

Figure 1. Combined Effects of Temperature and CO2 on %AET/PET Ratios. Wmax=0.88 mm/day.

Figure 2. Combined Effects of Temperature and CO2 on Soil Moisture Depletion Curves. Wmax=0.88 mm/day.

Figure 3. Combined Effects of Temperature, CO2, and Available Water on Agricultural Benefits. Wmax=0.88 mm/day.

The combined impacts of temperature and CO2 are very important when considering that temperature changes are uncertain as to their 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 change scenarios analyzed. Analysis of the single climate change scenarios of temperature and CO2 can be found in Ramirez and Finnerty (1995a).

Water Table Fluctuations

A shallow water table exists at the study site, ranging from 1 to 4 meters below the soil surface (Yenter et al., 1980). Water table variations may be caused by natural dynamics in groundwater recharge, or from irrigation pumping schedules and other consumptive uses. The contribution to root zone soil moisture from a shallow water table increases as the depth to the water table decreases and is a major component in the soil water balance. Table 2 illustrates the effect of water table fluctuations on the maximum rate of capillary rise, Wmax. The effect of capillary rise on soil moisture depletion curves is illustrated in Figure 4. As the soil moisture content decreased the capillary potential in the soil increased, and consequently so did capillary rise. The depletion curves exhibited asymptotic behavior, converging in time to a soil moisture concentration where capillary rise was balanced by actual evapotranspiration. The same behavior was found for AET functions, converging on the point where AET of water 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 water table reduced crop water stress and increased crop yields. Capillary rise was most important to agriculture when there is limited irrigation water because capillary rise substitutes for irrigation 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 use requirements.

Table 2: Capillary Rise with Depth to Water Table

DEPTH, cm 100 150 200 250 300
Wmax, mm/d 4.44 1.29 0.54 0.27 0.16

Figure 4. Effects of Depth to the Water Table Z on Capillary Rise and Soil Moisture Depletion.

Precipitation Changes

The high variability of the spatial distribution of global precipitation causes a large degree of uncertainty concerning regional and local precipitation changes resulting from various temperature and CO2-induced climate change scenarios. There is also a lack of understanding of potential changes in rainfall characteristics such as storm intensity, duration, and arrival rates. The impact assessment of precipitation changes on agriculture analyzed the following scenarios: both a 25% increase and decrease in storm intensity, storm duration, and storm arrival rates. For the analysis precipitation characteristic changes were made independently, while holding all other processes constant. The precipitation changes were applied to the entire growing season for all homogeneous precipitation periods, and were shown to impact soil moisture dynamics and agricultural benefits.

The results obtained from the analysis showed agriculture to be relatively insensitive to climatic precipitation changes for the cases analyzed (Ramirez and Finnerty, 1995b). Precipitation provides a minor portion of crop water use requirements in the San Luis Valley's irrigated agricultural production. The valley's low precipitation quantity of 76 mm/acre/season is small as compared to the 381 mm/acre/season of irrigation water used. Varying precipitation by 25% (19 mm/acre/season) had little or no effect on irrigation water use requirements and agricultural benefits.


Doubling of atmospheric CO2 has been experimentally proven to increase crop yield and biomass production by 43% to 75% in potato crops (Collins, 1976). Root and tuber crops were found to increase marketable yield by an average of 52%, as observed in 17 experiments of doubled atmospheric CO2 concentrations (Kimball and Idso, 1983). This result shows that increasing CO2 will significantly increase maximum crop yields and expected agricultural benefits given the same production and irrigation costs that currently exist.

Actual evapotranspiration was reduced even when biomass and plant 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 of CO2 fertilization and crop water use efficiency. Case 1 is a 50% increase in crop yield, combined with the historical PET rate. Case 2 is a 50% increase in yield, coupled with the reduced 1.5xCO2 PET rate. The third case is a 50% increase in yield, coupled with the reduced 2xCO2 PET rate. The 1992 maximum potato yield was equal to 375 100lbs/acre, 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), resulted in significant increases in agricultural benefits for all cases of available irrigation water. The results illustrate how agricultural benefits almost doubled under doubled CO2 concentrations, due to a large increase in crop yield coupled with increased crop water use efficiency. CO2 fertilization could make agriculture economically feasible in regions with high production and irrigation costs, and where available irrigation water is constrained. In addition, CO2 fertilization had a much greater positive effect on agricultural benefits than the effects of CO2 on stomatal resistance and PET (Finnerty, 1994).

Table 3: CO2 Fertilization Effects on Agricultural Benefits ($/ac), Wmax=0.88 mm/day

HIST. ET 1.5xCO2 ET 2xCO2 ET
00 mm 370.1 555.2 640.8 723.9
300 mm 1365.3 2081.9 2109.5 2206.9


The laws of supply and demand establish crop market values, and influence production decisions concerning acres to be planted, irrigation water requirements, and production costs. These decisions are made prior to the start of the growing season when a large degree of uncertainty exists. Farmers are at financial risk due to the uncertainty associated with future crop market prices, natural weather variability, unforeseen production costs, and natural disasters. The 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 Valley, Colorado.

Production Costs

Production costs are difficult to predict at the beginning of the 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. These two objectives are in direct opposition because increasing spending on production generally increases crop yield. This makes it difficult to 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 were analyzed. These climate changes were made while assuming current atmospheric CO2 concentrations. The results of the analysis showed the maximum expected benefits for all climate change scenarios to be greater than the production costs, given sufficient available water (400 mm/acre/season) and a significant contribution of capillary rise (0.88 mm/day). However, a 10% increase in production costs removed all profits for all temperature and precipitation change scenarios analyzed. Conversely, a decrease in production costs, or an increase in production efficiency would increase profit margins (Ramierz and Finnerty, 1995b). A 10% increase in production costs had a larger impact on agriculture than +/-14% changes in PET or +/-25% change in precipitation.

Market Value of Crops

Farmers generally plan for an average crop market value at harvest time. However, the price of Colorado potatoes has fluctuated around the ten year mean of $4.65/100lbs, from a fifteen year low of $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 crop market values because of the excess supply at harvest time.

The results of the analysis on the impacts of crop market price variations on agricultural benefits are displayed in Table 4. The table shows a 50% increase in crop market price caused a very significant increase in agricultural benefits, while a 50% decrease in 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

0 mm 370.13 555.59 185.46 1315.00
400 mm 1427.81 2182.86 677.07 1315.00


1. All temperature and CO2 climate change scenarios had a favorable impact on evapotranspiration, soil moisture depletion, and irrigated 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 significantly impacted the contribution of capillary rise to root zone soil moisture and agricultural benefits.

5. Agricultural sensitivity to agro-economic parameters had a larger impact on agricultural benefits than any of the climate change scenarios analyzed.

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 water use requirements had to be met either from precipitation, capillary rise, or irrigation. Irrigation was capable of supplementing any reduction of soil moisture caused by increased PET, lowering of the water table, or decreased precipitation. However, those irrigation water resources needed to be available to reduce agricultural risks attributed to damaging climatic or economic conditions if agricultural production was to remain profitable in the region.


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