Sensitivity of some potential evapotranspiration estimation methods to climate change

Sensitivity of some potential evapotranspiration estimation methods to climate change

AGRICULTURAL AND FOREST METEOROLOGY ELSEVIER Agricultural and Forest Meteorology 77 (1995) 121-125 Comment Sensitivity of some potential evapotrans...

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AGRICULTURAL AND FOREST METEOROLOGY ELSEVIER

Agricultural and Forest Meteorology 77 (1995) 121-125

Comment

Sensitivity of some potential evapotranspiration estimation methods to climate change S. Jeevananda Agricultural

Meteorologist

(Managing Colony,

Consulrant,

Phase-l,

Reddy

Jeevan Agromet

Secunderabad

Consultancy),

Plot No. 6, KRISAT

500 009, A.P.. India

Received 1 August 1994; accepted 14 March 1995

Abstract Climatic change and its impact on environment, and thus the consequent effects on human, animal and plant life, is a hot topic for discussion at national and international forums both at scientific and political levels. However, the basis for such discussions are scientific reports. Unless these are based on sound foundation, the consequent effects will be costly to exchequer. The paper of McKenney and Rosenberg (McKenney, MS. and Rosenberg, N.J., 1993. Sensitivity of some potential evapotranspiration estimation methods to climate change. Agric. For. Meteorol., 64: 81-I IO.) is looked into in this context.

1. Comments McKenney and Rosenberg (1993) evaluated eight potential evapotranspiration models’ ‘sensitivity’ to probably expected climatic change induced effects on meteorological parameters that are inputs into those models. Potential evapotranspiration (ETp) is an estimated meteorological parameter and relates to one or more meteorological parameters, such as radiation, temperature, relative humidity, wind speed, etc. (although radiation, in many cases, has to be estimated through other meteorological parameters). ETp values are derived using mathematical functions that integrate meteorological parameters and constants are derived through statistical analysis. The constant(s) so derived through statistical analysis are location/data specific in the majority of cases. The eight equations that were used in the study are quite different from one another both in terms of mathematical form and input meteorological data requirements. With this in mind let us now scan through the paper of McKenney and Rosenberg (1993): 0168-1923/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168- 1923(95)02239-2

In the McKenney and Rosenberg study, the percentage changes in ETp attributed climate change can also be attributed (partly) to scientists’ induced factors, such as the choice of ETp model and ETp model vs. environment; (ii) probable changes meteorological parameters due to climate change, expressed as absolute change percentage change; and (iii) ETp changes expressed in terms of absolute change percentage change.

to (i) in or or

1.1. Models

It is a fact that in the literature there are many ETp models. However, the selected model, even if it is in wide use, should be accurate enough to estimate ETp under diverse environmental conditions (at least for the 5 locations used in the study). Otherwise there is every possibility that the errors introduced by the model may mask or completely change the climate change induced effects. (Moreover, the accuracy of the outputs from a reliable water balance model relate to the accuracy of ETp values used in the analysis as the other input, rainfall, is an observed parameter). Also, many times the under- or over-estimation of ETp by any given model relate to climate regime in which the model is applied. Some models are less affected than others. Thus, unless these are demonstrated clearly, the conclusions drawn on the impact of climate change on ETp estimates of a given model have little significance. Therefore, the first question one should ask is whether any of these models are suitable to such an analysis? The answer to this depends on the accuracy with which the model can suitably estimate ETp under wide climatic conditions. The authors attempted no such comparisons in their study, except for a few statements from the air. If the Penman-Monteith method is the best, then why do all the labour? And, why then fill the pages with the quite obvious? There are publications that contrast this theory either. As the authors are not able to prove the accuracy of the models, what purpose does it serve to apply these to further theories and then draw conclusions. See, for example: “Although temperature-based methods are useful when data on other meteorological parameters are unavailable, the estimates produced are generally less reliable than those which take other climatic factors into account” (page 82); “Because ETp was not measured at the five locations for the entire year, direct comparison of values predicted by the eight methods with measured values is not possible. However, we will assume the Penman and Penman-Monteith methods to be the most reliable because these methods are based on physical principles and because they consider all the climatic factors which affect ETp” (page 92). If this is so, then why present comparisons based on purely hypothetical statements. “We have less confidence in the Penman than in the Penman-Monteith results because the former method relies on an empirical wind function which takes many different forms in the literature” (page 92). Do the ETp estimates presented in the paper support this observation in any way? “Unfortunately these values are not strictly comparable among methods because of a lack of standardization in the literature concerning how the computed quantities are defined” (page 89). Why is there a lack of standardization? Does the definition of ETp differ for different models? See, for example, FAO (1984).

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and Forest Meteorology

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“These differences should not affect our analysis of climate sensitivity, but they do limit the extent to which we can draw conclusions” (page 89). If this is true, then what purpose does it serve? ETp values used in water balance models have a unique definition (and no two definitions exist in the literature). Therefore, standardizing the model estimates to the unique definition can overcome some of the superfluous percentage changes in ETp, see Figs. 2 and 3 and Figs. 4 and 5. 1.2. Parameter-

magnitude

or percentage?

e-actual vapour pressure, T-temperature, Rs-solar radiation, RH-relative humidity and W-wind speed. The second question one should ask is to express the impact of climate change related changes in meteorological parameters, such as T, RH, Rs and W on ETp. Which is the correct procedure: is it the magnitude of change or is it the percentage of change, or is it that in some cases the magnitude of change and in others the percentage of change. In the present study the later course was adopted: “Sentivity to temperature change was then assessed for each of the methods by varying temperature over a range of 0 to +6”C in 2°C increments.” For Rs, RH and W, sensitive analyses were performed for each of the methods that require such inputs. A range of +30% in each variable was tested using increments of 10% (page 88) and thus the results reflected accordingly. For example: In the case of T, the magnitude of change was considered while in the case of Rs, RH and W the percentage changes were considered. That is, the magnitude of change in Rs, RH and W are quite different at different values of these parameters. This means that at lower ranges of Rs, RH and W he 30% change contribute to lower values, while at higher range, at 30% change, it contributes to higher values; in the case of T, the range of T is not important. Let us look at this with reference to the lowest and highest values of T, Rs, RH and W given in Table 4 (the mean climatic conditions in 1985): Parameter

Lowest value (range)

Highest value (range)

Rs (MJm-*) RH (%) W (kmday-‘1 T (“Cl

154 (108-200) = 72 57 (40-74) = 34 134 (94-174) = 80 4.6 (4.6-10.6) = 6

191 (134-248) = 114 74 (52-96) = 44 287 (201-374) = 174 16.2 (16.2-22.2) = 6

Range refers to the magnitude of a parameter at - 30% to + 30% level; while in the case of T it is T to T + 6 only. Obviously, the percentage effect of Rs, RH and W on ETp are quite different to that of T, based on their magnitude at a given location/season. If the magnitude of change would have been used, like in the case of T, then the conclusions drawn might have been quite different. It is simple to say that this is the standard procedure, but the consequent impacts on the conclusions drawn are quite different. Thus, such analysis may not reflect the real changes that are associated with climate change, but quite often reflect the scientists’ induced changes. If T can not be expressed as percentage change, then why not express

the changes in Rs, RH and W in absolute terms? It is also a fact that the effect of climate change on T is quite different under different T regimes. Then how is it justified to use fixed change in absolute terms rather than percentage change? It is. in fact, possible to express the changes in T as percentage with minor restrictions on the range of T. I wonder, how can one explain the ‘marriage of convenience’ approach followed in the case of KH (see page 108) where the magnitude change in T interacts with the percentage change in e, and at the same time translating e and T into KH in models where this is an input. 1.3. ETp-

magnitude

or percentage?

The third question one should ask is if, from the water budget point of view (as stated in the paper), the percentage change of ETp or the magnitude of change of ETp is relevant. McKenney and Rosenberg adopted the former: the values are shown as percentage change (page 92). Thorntwaite produces the lowest annual values at all locations and Penman the highest differences between the two in excess of 100% at some sites (page 89). On page 95, McKenney and Rosenberg state that “the change in ETp is given as percentage, while the change in T is given in absolute terms. (Because temperature is measured as an interval scale, not a ratio scale, a percentage in temperature is meaningless.) Since a one degree change in temperature is, in a relative sense, a greater warming at a cool location than at a warmer one”. The same argument can also apply to ETp, in specific, and to other meteorological parameters, in general. What is the problem in expressing even ETp in absolute terms. If this is done, the majority of the differences seen in pages 93 and 94 will disappear. As the ETp is model specific, if the estimates of a given model (like Thornthwaite, 750 mm) are lower than another (Penman, 1500 mm), then for the same magnitude of increase or decrease (150 mm) as a consequence of supposed climatic changes, expressed in percentage change of ETp, naturally they present quite a different picture (20% and 10%). The magnitude of the ETp vs. the ET model presented in Figs. 2 and 3 are quite obviously reflected in the results presented in Figs. 4-6. If the magnitude of change, which is important in the water budgeting study, had been used, then the conclusions drawn would have been quite different. 1.4. Climatic change The fourth question one should ask is if the supposed climate change induced changes in meteorological parameters, as noted in this study, are in line with the observed facts. McKenney and Rosenberg state that “the increasing concentration of CO1 and other radiatively active gases.. . is expected to cause tropospheric temperatures to rise. Changes in other climatic elements, such as precipitation, cloudiness, humidity, and windiness, are likely to follow changes in temperature” (page 81). “Another way in which increased atmospheric CO, may influence water availability is through the direct

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und Forest Meteorology

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effect on CO, on vegetation. Experiments in controlled environments have demonstrated that increased ambient CO, acts to stimulate plant growth and to increase stomata1 resistance, although there is some question as to whether or not these effects will occur under field conditions” (page 83). Hemispheric and global surface temperature trends based on world climate observation networks suggest a moderate, about one degree Celsius, change in temperature during 1860-1988 (WMO, undated). A WMO/UN Fact Sheet (WMO, undated) suggests that global mean surface temperatures will rise by the order of 2°C to 5°C with a doubling of CO, equivalent in the atmosphere. Such a doubling under the impact of human actions is expected to occur by about 2030 to 2050, depending on control measures undertaken. Sarkar and Thapliyal (1988) observed that long term rainfall records from both the hemispheres show similar small effects, but no clear trends emerge on a hemisphere or global basis. It can therefore be summarized that recent impact of climate on human economy has been largely due to variability, rather than to lasting climate change (in relation to rainfall). McKenney and Rosenberg state on page 105 that if this is the case. are the differences in ETp predicted by the various methods then inconsequential’? Not necessarily. To answer this let us see their own results: The results in Table 5 suggests that the average annual change in climate response to doubled atmospheric CO,, as predicted by the GISS and GFDL general circulation models (which is accurate?), are quite different (GISS predicted values are less than those of GFDL), particularly in the case of Rs. It can be seen from Table 7, with reference to the Penman and the Penman-Monteith models, that the percent change in ETp in response to the GFDL-derived scenario of climatic change in all parameters and potential CO,-induced changes in vegetation characteristics are ‘negligible’, which are less than those expected from the model induced errors, at all the five stations ( < 6%). Are the GISS results (Table 6) valid as they could not predict any change in Rs? Also, the authors have not tried to verify, at least, the changes reported in Table 5 with the observed data for the five locations, let alone at global or hemisphere level. 2. Conclusion McKenney and Rosenberg, fully with some qualitative justifications, presented the results. In their paper can be attributed, to a large extent,

aware of the lacunae and then simply ran the supposed changes to scientists’ induced

of their study, tried to justify the standard programmes and in ETp due to climate change factors.

References McKenney.

MS.

and Rosenberg,

methods to climate change.

N.J.,

1993.

Sensitivity

Agric. For. Meteorol..

FAO (Food and Agriculture Organization),

of some potential evapotranspiration

estimation

64: 8 I - 1 10.

1984. Crop water requirements. FAO Irrigation and Drainage paper

24. Rome, Italy, 144 pp. Sarkar, R.P. and Thapliyal, WMO

(World

V., 1988. Climatic change and variability.

Meteorological

sphere. Fact Sheet, WMO,

Organization). Geneva.

Museum, 39: 127- 138.

undated. Climate change and understanding the global atmo-