Is Water A Sustainable Resource? A Perspective from Evapotranspiration

 

 

Introduction

 

 

A renewable resource is one, that the usage of that resource does not threaten its availability in the future [1]. The hydrological cycle projects water as a component of an endless and continuous cycle, reflecting it as a never vanishing resource. Therefore, the implicit concept behind hydrological cycle can be interpreted as water being a sustainable resource [2, 3]. Here, I will question this concept behind hydrological cycle in perspective of evapotranspiration.

 

 

The problem can be visualized from an animation crated by the Climate Lab Section of the Environmental Change Research Group at the Department of Geography of the University of Oregon [4]. This animation basically draws the picture of the net gain or loss of water on a global scale, provided if the gain of water is only by precipitation and loss of water is only by evaporation (Figure 1) [4]. As we can notice from Figure 1 there is no uniform net gain or loss or balance of water, throughout the whole world. As a fact, from Figure 1, some places always gain water, some places always in a loss, and of course, in some places there are balances [4]. However, with an eye approximation from Figure 1, there may appear a net balance of water on a global scale [4].

 

 

 

Figure 1. (Source: reference no. 4). Animation created by Climate Lab Section of the Environmental Change Research Group,  Department of Geography, University of Oregon, showing the net water gain/loss on global scale.

 

 

 

In order to study on the possibilities of using rain water for irrigation Droogers et al. (2001) created a global map that compares the regions with 75% probability for precipitation against potential evapotranspiration, on a global scale (Figure 2) [5]. Figure 2 shows that often time regions with chance of receiving lesser amount of water as precipitation are actually losing a greater amount of water as evapotranspiration [5]. As a matter of fact the driest regions in the world produce about 3000 mm per unit area of water lost by evapotranspiration in a whole year [5]. Therefore, Figure 2 is a good evidence to support the fact that regions losing more water by evapotranspiration are not necessarily receiving more water as precipitation [5]. Figure 1, also, supports the same fact [4]. Therefore, considering Figure 1 [4] and Figure 2 [5], we can easily comment that water may be a sustainable resource on a global scale, but it cannot be considered as a sustainable resource for a small region.

 

 

Figure 2. (Source: reference no. 5) Global maps for regions with probability of precipitation more than 75% (top figure) and for reference evapotranspiration rate (bottom figure), from Droogers et al. (2001).

 

 

Evapotranspiration is a phenomenon through which a large amount of water is lost, and therefore, it constitutes a major role in hydrologic cycle. An important step behind an efficient water management, for a certain region, is a fairly accurate estimation of evapotranspiration [6]. Rana and Katerji (2000) reviewed different evapotranspiration measuring technique elaborately [7]. They categorized different techniques involved with evapotranspiration estimation/measurement, depending on whether that particular procedure estimates or measures evapotranspiration. Thereafter, they further classified the two major groups into several subgroups, depending on the concepts/principles involved behind those procedures [7].

 

 

ET Measurement Procedures

 

 

It was probably idealized by Rose and Sharma (1984) [8] to group all the ET measuring and estimating techniques according to the scientific laws they utilize (cross referencing from Rana Katerji, 2000 [7]). Rana and Katerji (2000) adopted their [8] idea to classify all these procedures [7]. Rana and Katerji (2000) grouped the procedures that were built upon the concept of mass balance, as “Hydrological Approaches” (p. 128) [7]. Often water vapor is considered as another gas, and thereby, some ET measuring/estimating procedures have been developed from the consideration of energy and mass transport in gas [9]. Procedures involving gaseous properties of water vapor were grouped under the section of “Micrometeorological Approaches” by Rana and Katerji (2000) [7] (p. 128). Similarly, Rana and Katerji (2000) grouped the procedures that depend of the physiological properties of plants as “Plan Physiological Approaches” (p. 128), and for obvious reasons “Analytical Approaches” (p. 128) and “Empirical Approaches” (p. 128) in their own groups [7].

 

 

In the following sections (before Case Study) little description of a few important ET estimating procedures are given.

 

 

Weighing Lysimeter

 

 

A weighing lysimeter is basically a container occupied by vegetation(s) of interest. The basic concept behind the working principle is that any change in the water content of the whole system, due to ET, would reflect as a change in the mass of the whole system [7].

 

 

 

 

Energy balance equation

 

 

A fraction of the total amount of solar energy, that land surface receives, are radiated back. If the fraction that is radiated back is deducted from the total amount of solar energy received, what is left with is called the net radiation [9; 10]. The total energy involved with net radiation are generally distributed into three components, the sensible heat energy, the latent heat energy, and the soil heat flux. The latent heat energy constitute to ET by changing water from liquid into vapor phrase. This means if other three components can be measured accurately, latent heat energy can be measured using simple mathematics [7; 9; 10].

 

 

Eddy Covariance Method

 

 

The instantaneous measure of any variable that is fluctuating with time can be represented as the mean of the variable, over an appreciable period time, plus the fluctuation from that mean at that instant. If there is no net gain or loss of that particular variable, then over an appreciable period of time these fluctuations sum up to zero. However, if we consider the product of two such variables, then the product of the fluctuations of the two variables does not add up as zero, even though there is no net gain or loss of any of those two variables. Applying laws of physics and mathematics it can be shown, that inside a parcel of air when the product of such vertical fluctuations of moisture flux and wind flux are considered it gives the estimate of latent heat energy, which is the coefficient of latent heat energy times ET [9].

 

 

 

 

Bowen Ratio method

 

 

The concept of Bowen ratio was introduced by I. S. Bowen [12], which is simply the ratio between sensible heat to latent heat [7; 9; 10]. The importance of this idea is that the Bowen ratio, represented by the Greek letter “beta” (β), can be easily computed from the ratio of the difference of air temperatures measured at two different elevations to the difference of air humidity measured at two different elevations. Thereafter, applying mathematical equations latent heat energy can be estimated [7; 9; 10].

 

 

 

 

Sap Flow Method

 

 

The concept of sap flow stands on the concept that a plant’s transpiration rate and sap flowing through its step are related to each other [7].

 

 

 

 

Crop Coefficient Method

 

 

Crop coefficient method is basically a comparison analysis. ET is estimated for a certain vegetation type (commonly known as reference vegetation) and a comparison factor (commonly known as crop coefficient) is used to estimate ET for other vegetation types [7; 10; 12]. U.S.B.R. uses crop coefficient approach to estimate ET for their reach areas [13; 14]. They use satellite image to classify vegetation according to different vegetation types and uses crop coefficient to estimate ET rate from different vegetation types [13; 14]. Thereafter, they multiply the ET rate with the total area that those vegetations occupy, and calculate volumetric water loss by ET from their reach areas [13; 14]

 

 

SEBAL

 

 

The Surface Energy Balance Algorithm for Land (SEBAL) was developed by Bastiaanssen et al. (1998a) [15] and validated by Bastiaanssen et al. (1998b) [16]. The algorithm first develops maps of hemispherical reflectance, surface temperature, and normalized difference vegetation index (NDVI) maps from satellite images. Thereafter, these maps are used to develop different components of the energy balance equation, stated above. Likewise, the latent energy is calculated using simple mathematics when the rest of the components of the energy balance equation are estimated [15].

 

 

Empirical Model by Nagler et al. (2005)

 

 

Nagler et al. (2005) developed an empirical model to estimate ET from riparian vegetation of the Rio Grande River in New Mexico [17]. They [17] used eddy covariance data (previously published Cleverley et al. (2002) [18] and Dahm et al. (2002) [19]) as their response variable and maximum air temperature and enhanced vegetation index (EVI) as regressor variable [17]. EVI is a unit less parameter that actually quantifies the stress level in the plant [20].

 

 

 

 

 

 

Case Study (Palo Verde Irrigation District, California)

 

 

My research area is Palo Verde Irrigation District (PVID) in California. The following table summarizes how much water PVID is losing by ET (all data used in the table are from U.S.B.R. (2007 [13]; 2008a [14]) as mentioned inside the table:

 

 

Calendar Year	Agricultural Acreage (acres)	Total Agricultural ET (acre-feet)	Riparian Acreage (acres)	Total Riparian ET (acre-feet)	Data and Information Source
2006	69388.72	320830	2455.43	8691	U.S.B.R. (2007)
2007	75909.12	347880	2381.52	8554	U.S.B.R. (2008a)

 

 

 

 

 

 

In order to understand how much of water was lost as ET let us look at the following table developed using data and information from U.S.B.R. (2007 [13]; 2008a [14]; 2008b [21]; 2008c [22]) as indicated inside the table:

 

 

 

 

 

 

Calendar Year	Total Agricultural ET (acre-feet)	Total Riparian ET (acre-feet)	Total ET (agricultural + riparian) (acre-feet)	Water Consumptive Use by PVID (acre- feet)	Percentage of Water lost by ET (Total ET/Consumptive Use)*100
2006	320830 U.S.B.R. (2007)	8691 U.S.B.R. (2007)	329521	354898 U.S.B.R. (2008b)	92.85%
2007	347880 U.S.B.R. (2008a)	8554 U.S.B.R. (2008a)	356434	375347 U.S.B.R. (2008c)	94.96%

 

 

The above tables are self evident how much water is being lost from PVID, just by ET. As a result U.S.B.R estimates ET for every calendar year to help manage water in their reach areas [13; 14]. PVID consists of small town and mostly agricultural region [25]. Therefore, from the above table it is quite clear that most of the water consumed by PVID were actually lost by ET during 2006 and  2007.

 

 

Policy and Management

 

 

PVID is dominantly an agricultural region. The total area of PVID is approximately 131298 acres, and most of which are agricultural region. The water supply to this region comes from Colorado River. The predominant usage of water is for irrigation purpose. The two main irrigation crops are alfalfa, and cotton. So the main stake holders for the water supplied to this region are people associated with irrigation [25]. Boronina et al. (2005) [26] described the importance of a proper estimation of ET for an accurate water management, even for a semiarid region in Cyprus. For them estimating actual ET was a harder task as there were times when actual ET went very close to potential ET or in other words maximum possible ET in that given climatic condition. Jensen and Wright (1978) [27] described the importance of estimating ET for proper scheduling of irrigation. To design an appropriate irrigation schedule different information are needed to be considered, such as crop planning time, depth of water table, etc., and most certainly water lost by ET [27]. In order to meet this last criterion U.S. Bureau of Reclamation began routine estimation of ET even back in 1976 [27].

 

 

From the concept of hydrological cycle it is clear that water loss by ET is an unstoppable loss of water [2, 3]. To manage the water demand for agriculture, posed by PVID and Metropolitan Water District (MWD) near Los Angeles, CA, PVID and MWD reached an agreement between them. As a part of this 35 year agreement farmers in PVID may participate voluntarily. They fallow some agricultural lands. During this fallowing time they do not irrigate, but just maintain their land. The water saved this way is diverted to MWD. That is how they encounter the water lost by ET [23, 24].

 

 

 

 

 

 

Importance of ET estimations in different situations (Global outlook)

 

 

Droogers et al. [5] developed a global map of evapotranspiration (Figure 2). From Figure 2 it is quite evident that water loss by evapotranspiration is global phenomenon. In the following section a brief discussion have been given about how ET estimation became a concern for scientific works in different climatic conditions, from recent scientific articles:

 

 

DeTar (2009) [28] worked on semi-arid region of San Joaquin Valley in California. Although they mainly worked on developing a proper crop coefficient for cowpea plants, yet their main intension was to betterment of water management. They determined crop coefficient for cowpea plants for different reference ET.

 

 

Pollok et al. (2009) [29] conducted their study in a sub-humid region in New Zealand. Their main goal was to develop a proper water management for pasture and silvopasture lands near a place named Lincoln in New Zealand. They estimated ET by applying the concepts of soil water balance method.

 

 

Roy et al. (2009) [30] intended to develop a software that will guide the users to develop a reservoir in the agricultural farm that will store the rain water, which can be used for irrigation during comparatively dry periods. Their study site was in a humid part of India. Even in this type of situation they considered water loss by ET into their calculation, in order to perform a proper water balance analysis. They used crop coefficient method to estimate ET.

 

 

Hess et al. (2009) [31] studied in a semi-arid region in Botswana; but in addition aridity of the climate the region suffered from an additional problem of water salinity. Their aim was to develop water management of cabbage farms. Even in their study they accounted for water loss by ET into their model. They estimated ET using crop coefficient approach.

 

 

 

 

Conclusion

 

 

From the above discussions it is quite evident that a proper estimation of ET plays an important role in water managements for various situations- say it is arid [13; 21], semi-arid [28; 31], sub-humid [29], or even humid conditions [30]. Again, as global warming is a rising concern for many intellectuals (see Chip’s Assignment [33]), Henderson-Sellers et al. (2008) [34] argued in favor of incorporating ET into the calculation while making predictions of futuristic climates and environments. Even while studying water management for forests Vanclay (2009) [32] pointed out the “need” of a better understanding of the mechanism of ET and its importance in water management. Above all, the major problem is that different ET estimating procedures have different drawbacks [7; 10]. Therefore, it is true, from the above discussions, that a proper estimation of ET is essential for water management for any region; however, there is not a unique ET estimating technique that can be applied universally to any case [7; 10]. In my research I am using satellite sensors to develop empirical models that would estimate ET for PVID, as accurately as possible.

 

 

 

 

 

 

References:

 

 

 

 

 

 

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