Rinnovo di superficie: un metodo avanzato micrometeorologici per la misurazione e l'elaborazione del campo scala dell'energia densità di flusso di dati

Summary

Surface renewal is a micrometeorological method that is being used increasingly to determine energy fluxes, but its technical complexity makes it inaccessible to a broad audience. We describe the steps needed to set up and calibrate a surface renewal field station, to acquire and process data, and to correctly interpret results.

Abstract

Advanced micrometeorological methods have become increasingly important in soil, crop, and environmental sciences. For many scientists without formal training in atmospheric science, these techniques are relatively inaccessible. Surface renewal and other flux measurement methods require an understanding of boundary layer meteorology and extensive training in instrumentation and multiple data management programs. To improve accessibility of these techniques, we describe the underlying theory of surface renewal measurements, demonstrate how to set up a field station for surface renewal with eddy covariance calibration, and utilize our open-source turnkey data logger program to perform flux data acquisition and processing. The new turnkey program returns to the user a simple data table with the corrected fluxes and quality control parameters, and eliminates the need for researchers to shuttle between multiple processing programs to obtain the final flux data. An example of data generated from these measurements demonstrates how crop water use is measured with this technique. The output information is useful to growers for making irrigation decisions in a variety of agricultural ecosystems. These stations are currently deployed in numerous field experiments by researchers in our group and the California Department of Water Resources in the following crops: rice, wine and raisin grape vineyards, alfalfa, almond, walnut, peach, lemon, avocado, and corn.

Introduction

Water scarcity threatens agricultural sustainability in dry growing regions. This situation will likely worsen with changing climatic conditions and increasing competition between agricultural, municipal, industrial, and conservation entities for limited water supplies. To deal with this ongoing dilemma, growers continually search for ways to improve irrigation efficiency, and technology that better estimates crop water use (i.e. evapotranspiration, ET) in real time, and better agrometeorological methods will certainly help these efforts. Current technologies used by growers to estimate ET depend on the use of reference evapotranspiration (ET₀) and an empirical crop coefficient (K_c), both of which are highly susceptible to estimation errors. Agrometeorologists also measure ET to evaluate experimental treatments for improving crop water use efficiency¹ and to parameterize regional water allocation strategies², but these methods are highly technical and expensive. Efforts are currently underway to translate existing research-based ET measurement methods into cost-effective and user-friendly technologies for growers.

Surface renewal (SR) is one agrometeorological method used to measure crop ET. SR is based on analyzing the energy budget of air parcels that reside ephemerally within the crop canopy during the turbulent exchange process³ as measured with the station shown in Figure 1 and illustrated theoretically in Figure 2 and Movie 1. The air parcels are manifested as ramp-like shapes in turbulent temperature time series data, and the amplitude and period of the ramps are used to calculate the flux density (Figures 2 and 3). With the SR method, ET for a given crop surface is determined by calculating latent heat flux density (LE) as the residual of the following energy balance equation
LE=R_n-G-H,

Here, LE is the energy flux density associated with the phase change of water from a crop surface, R_n is the net radiation, G is the soil heat flux density (i.e. energy conducted into or out of the ground), and H is the sensible heat flux density (i.e. energy flux density from the surface to the air or vice-versa that results in a temperature change). R_n is a positive number when the net flux is downward (energy added to the surface), LE and H are positive numbers when the flux is upwards (energy added to the air), and G is a positive number when the flux is downward (energy added to the soil). LE (MJ/m²sec) is then divided by the latent heat of evaporation (L=2.45 MJ/kg) to obtain the mass flux density of water vapor from the surface (i.e. ET).

Measurements of R_n and G are relatively straightforward and inexpensive. Direct measurements of H are more complex and require high frequency data acquisition. The most common method to determine H is with eddy covariance; however, the sonic anemometer required for this method is expensive, complex and, consequently, not widely used by agronomists, horticulturalists, or engineers to determine H and LE. On the other hand, H derived from the SR technique is obtained by a simpler and less expensive method, which uses fine wire thermocouples to measure high frequency air temperatures at the surface-atmosphere interface. Despite the simplicity of the SR, current measurements still require calibration against a sonic anemometer's eddy covariance estimate of H to obtain sensible heat flux density.

The successful deployment of an SR flux tower for ET measurements can be a daunting challenge, especially for many agricultural researchers without formal training in atmospheric science. SR and eddy covariance measurements require sophisticated technical skills in both programming data loggers to execute tasks with complex instrumentation and writing computer programs to post-process the raw turbulence data into meaningful fluxes. Here, we describe how to setup a field station, install sensors, and utilize our new turnkey data logger program for data collection and post-processing procedures.

Protocol

Perform a wind direction and footprint analysis prior to field installation; this step is important to insure that the tower is situated properly to capture the representative flux in the study field. Before establishing the field station, you must evaluate the prevailing wind direction in the study plot, and determine how much fetch (upwind distance) is needed for your particular application. The distance of the fetch is dependent on the height of the surface to be measured and the height of the…

Representative Results

Acquiring the Rn and G components of the energy balance equation are relatively straightforward and inexpensive. Measurements of H are more complex and require high frequency temperature traces measured with a fine wire thermocouple for surface renewal or a sonic anemometer for eddy covariance. An example temperature trace measured with a fine wire thermocouple is represented in Figure 9, and it shows the need for mathematical analysis to extract the signal from this…

Discussion

The surface renewal method presents a tangible opportunity to develop a stand-alone and inexpensive technique to quantify crop water use in real time. Recent advances in signal processing, data logger programming, calibration, and data management have brought this goal into clearer focus. This manuscript and our recently developed turnkey data logger program⁶ render advanced micrometeorological methods more accessible for agricultural researchers. The output table from the turnkey data program that contains th…

Materials

Equipment/Material	Company	Catalog Number
Datalogger	Campbell Scientific, Inc.	CR1000
Datalogger Enclosure	Campbell Scientific, Inc.	ENC12/14-SC
Tower (6 ft) with grounding & lightning rods	Campbell Scientific, Inc.	CM6
Cross arm (4 ft)	Campbell Scientific, Inc.	CM204
Nu-rail crossover fitting	Campbell Scientific, Inc.	17953
Power supply (12 V) with regulator & battery	Campbell Scientific, Inc.	PS100
Charger Regulator (12 V)	Campbell Scientific, Inc.	CH100
Battery Extension Cable	Campbell Scientific, Inc.	6186
Thermocouple Extension Cable	Campbell Scientific, Inc.	FWC-20
Thermocouples	Campbell Scientific, Inc.	FW3
3D sonic anemometer with long neck	RM Young Company	81000 RE
8 conductor cable for anemometer	RM Young Company	18660
Soil heat flux plates	REBS, Inc.	HFT3.1
Soil thermocouples	Campbell Scientific, Inc.	TCAV
Net radiometer with cable	Kipp and Zonen, Inc.	NR Lite 2
Heavy duty pole mount for radiometer	Kipp and Zonen, Inc.	L-CMB1