Accounting for vegetation height and wind direction to correct eddy covariance measurements of energy fluxes over hilly crop fields

Abstract

As agricultural hilly watersheds are widespread throughout the world, there is a strong need for reliable estimates of land surface fluxes, especially evapotranspiration, over crop fields on hilly slopes. In order to obtain reliable estimates from eddy covariance (EC) measurements in such conditions, the current study aimed at proposing adequate planar fit tilt corrections that account for the combined effects of topography, wind direction, and vegetation height on airflow inclinations. EC measurements were collected within an agricultural hilly watershed in northeastern Tunisia, throughout the growth cycles of cereals, legumes, and pasture. The wind had two dominant directions that induced upslope and downslope winds. For upslope winds, the airflows were parallel to the slopes and slightly came closer to the horizontal plane when vegetation grew. For downslope winds, over fields located in the lee of the rim top, the airflows were almost horizontal over bare soil and came closer to the topographical slope when vegetation grew. We therefore adjusted the planar fit tilt correction on EC measurements according to vegetation height and by discriminating between upslope and downslope winds. This adjusted tilt correction improved the energy balance closure in most cases, and the obtained energy balance closures were similar to that reported in the literature for flat conditions. We conclude that EC data collected within crop fields on hilly slopes can be used for monitoring land surface fluxes, provided planar fit tilt corrections are applied in an appropriate manner.

Observing Actual Evapotranspiration within a Hilly Watershed: Case Study of the Kamech Site, Cap Bon Peninsula, Tunisia

“…First, the combined effects of hilly topography and wind direction induced changes in airflow streamlines and turbulent fluxes [23]. Second, when observing fluxes at the field scale in the same study area, [21] and [22] reported the existence of two dominant wind directions, and demonstrated the necessity to discriminate these two wind directions when processing the EC data. Thus, the gap filling methods are likely to provide different relationships between ancillary information and flux measurements, when considering different wind directions.…”

“…These tests permitted to ensure that the theoretical requirements for the EC measurements were fulfilled [27]. On the same site, [21,22] applied these tests over EC datasets collected under conditions of hilly topography, and reported good energy balance closure for the selected data. We kept the high and good quality classes as defined by [28] and [29], since these two classes are considered as suitable for long-term observations.…”

“…Third, as the flux tower was located near the bottom of the northern rim of the Kamech watershed, NW winds induced descending air flows, whereas S winds induced ascending air flows. On the same hilly site, [21,22] highlighted the importance of considering the wind direction when conducting eddy covariance flux measurements. Therefore, the REddyProc gap-filling procedure was applied in two different manners: without or with discriminating between the two dominant wind directions (see section 2.5.2).…”

“…For hilly and heterogeneous cropping systems, the conditions that drive ETa (i.e., radiative, wind and turbulence regimes, vegetation patterns) can be different from those occurring for homogenous surfaces over flat / mountainous areas. For instance, it is necessary to adapt correction methods for EC measurements [21,22], or to account for footprint changes according to wind direction.…”

Replies to comments from the GI Editorial Review

“…The main potential drawback of the double rotation method is that a significant variability in rotation angles can be observed at low wind speeds (Turnipseed et al, 2003). Since our study area was typified by large wind speeds (Zitouna-Chebbi et al, 2012, 2015, we selected the double rotation method that is applied to each time interval over which the convective fluxes are calculated (30 min in our case). After a first rotation (yaw angle) that cancels the lateral component of the horizontal wind speed, a second rotation (pitch angle) is applied around a horizontal axis perpendicular to the main wind direction to cancel the mean vertical wind speed.…”