3. Conclusions

detect larger eddies better. Microwave scintillometers are more sensitive to humidity fluctuations than the other wavelengths, providing an inference to accurate ETa determination [35]. Infrared scintillometers have been used for some time; however, visible and microwave scintillometers are relatively new and have not been

Surface layer scintillometer (SLS-20, Scintec AG, Rottenburg, Germany).

Advanced Evapotranspiration Methods and Applications

Scintillometers have been considered to be very beneficial for ET remote sensing studies due to their large path length. Specifically, large aperture scintillometers (LAS), which can have path lengths up to a few km, can have a large enough spatial footprint to be similar to most remote sensing data resolution. In addition, the path averaging of the scintillometer provides an integrated benefit in that a homogenous surface is not required to meet any assumptions. This allows the scintillometer to be used across varying terrain and provide an averaged value. The averaging for variable surfaces is similar to that of remote sensing data. The previous points illustrate how the scintillometer can serve as a ground-truthing instrument or as a source of validation data for remote sensing. Since most of the ET remote sensing models are based on the surface energy balance, similarly to scintillometry, measurements other than just ET can be evaluated. The surface fluxes H and LE are determined by both scintillometers and ET remote sensing models, which provide more data for comparison. One benefit scintillometry has over EC is the lack of

One advantage SLS offers over other point source measurements is that the fluxes can be determined over shorter lengths and at heights closer to the surface [37]. In addition, the fluxes can be calculated on shorter temporal scales, as low as 1 minute, compared to EC, for example, which typically uses a 30 minute interval. An advantage the SLS has over the LAS is that the SLS determines the l0, which is proportional to the dissipation rate of the turbulent kinetic energy, ε, and CT

2 ,

used as extensively.

Figure 7.

corrections [36].

16

The methods mentioned above can all be used to determine ET; however, there are disadvantages to each one of them. With the soil water balance approach, the drainage and runoff terms can be difficult to determine. Although they are commonly miniscule in arid and semiarid regions, they would still need to be accounted for to obtain the greatest accuracy. Lysimeters are the most accurate but are very expensive and intrusive to install and operate. In addition, they require a high level of knowledge and experience to obtain the best measurements. The Bowen ratio method has been used to determine ET from the energy balance, but it is an indirect measurement. EC is a direct measurement method of turbulent fluxes but is known to have energy balance closure and other errors associated with it. Scintillometers are another indirect measurement method that has been extensively used, but they also have known errors. EC and scintillometers are two of the more common

turbulent flux and ET measurement instruments typically used. Remote sensing model studies are also widely available in the literature; however, each method, other than a lysimeter, typically has 20–30% error. For irrigation scheduling and some water resources management, 20–30% error may not be adequate, but for other purposes, such as modeling and land use change monitoring, the error may be acceptable. With many options available, the most suitable instrument/method will be dependent on the purpose and use of the ET data.

References

[1] Scanlon B et al. Impact of agroecosystems on groundwater resources in the Central High Plains, USA. Agriculture, Ecosystems & Environment. 2010;139(4):700-713

AgriLife Research; 2010

2009

Area; 2010

277-285

19

[3] Marek T et al. 2011 Panhandle Regional Water Plan Task 2 Report: Agricultural Water Demand Projections;

[4] Colaizzi P et al. Irrigation in the Texas High Plains: A brief history and potential reductions in demand. Irrigation and Drainage. 2009;58(3):257-274

[5] PRWPG, P.R.W.P.G. Regional Water plan for the Panhandle Water Planning

[6] Dutton AR, Reedy RC, Mace RE. Saturated Thickness in the Ogallala Aquifer in the Panhandle Water Planning Area: Simulation of 2000 through 2050 Withdrawal Projections. Bureau of Economic Geology. Austin, TX: University of Texas at Austin; 2001

evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome. 1998;300(9):87-210

[8] Howell T et al. Evapotranspiration of full-, deficit-irrigated, and dryland cotton on the Northern Texas High Plains. Journal of Irrigation and Drainage Engineering. 2004;130(4):

[9] Howell T et al. Crop coefficients developed at Bushland, Texas for corn, wheat, sorghum, soybean, cotton, and alfalfa. In: World Environmental and

[7] Allen RG et al. Crop

[2] Marek T et al. Assessment of Texas Evapotranspiration (ET) Networks. Vol. 379. Amarillo, Texas: Texas A&M

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

> Water Resource Congress 2006: Examining the Confluence of Environmental and Water Concerns;

[10] Evett SR et al. Evapotranspiration by soil water balance using TDR and neutron scattering. Management of Irrigation and Drainage Systems, Irrigation and Drainage Div./ASCE;

[11] Rockström J, Jansson PE, Barron J. Seasonal rainfall partitioning under runon and runoff conditions on sandy soil in Niger. On-farm measurements and water balance modelling. Journal of

[12] Univeristy of California at Davis, U.-D. Neutron Moisture Meters. 2015 [5/18/2015]; Available from: http:// ucmanagedrought.ucdavis.edu/PDF/ DROUGHT\_WEB\_NEUTRON\_PRB.pdf

[13] Allen RG et al. Evapotranspiration information reporting: I. Factors governing measurement accuracy. Agricultural Water Management. 2011;

[14] Evett SR et al. Can weighing lysimeter ET represent surrounding field ET well enough to test flux station measurements of daily and sub-daily ET? Advances in Water Resources. 2012;

[15] Marek TH et al. Design and construction of large weighing

[16] Schneider A et al. Hydraulic pulldown procedure for collecting large soil monoliths. Transactions of ASAE.

[17] Howell TA et al. Calibration and scale performance of Bushland weighing

ASAE. 1988;31(2):477-484

1988;31(4):1092-1097

monolithic lysimeters. Transactions of

Hydrology. 1998;210(1):68-92

2006

1993. pp. 914-921

98(6):899-920

50:79-90
