Estimating Small Grain Equivalents of Shrub-Dominated Rangelands for Wind Erosion Control L. J. Hagen, Leon Lyles ASSOC. MEMBER MEMBER ASAE ASAE ABSTRACT management systems. Current procedures for evaluating wind erosion equation, which estimates average wind erosion control practices utilize the wind erosion Aannual erosion, requires that all vegetative cover be equation (Woodruff and Siddoway, 1965). To use the expressed as dry biomass per unit area of flat small grain equation, one must express all vegetative cover in terms equivalent (SG)e. For a standing vegetative canopy, the of its equivalent to a small grain dry above-ground (SG)e depends on the magnitude of the friction velocity biomass reference standard (SO^. Prediction equations reaching an underlying erodible surface. The soil surface have been developed to predict (SG)e for several range friction velocity, and thus (SG)^, was shown to be a grasses (Lyles and Allison, 1980), as well as flat and function of aerodynamic roughness length of the canopy standing crop residues (Lyles and Allison, 1981). and the product of a drag coefficient and plant area However, (SG)^ prediction equations are lacking for index. Aerodynamic roughness, as well as canopy various shrub species to evaluate their ability to control silhouette area and mass distribution, were measured in wind erosion. sand sagebrush {Artemisia filifolia Nutt.) and yucca The standard procedure to evaluate (50)^ of plants is {Yucca elata Englem.) canopies. Estimating equations to conduct laboratory wind tunnel tests on the plants at were developed to predict (SG)^ of the sagebrush and various plant populations. Shrubs present special yucca canopies using either above-ground dry biomass or challenges, however, because many are too large to fit in plant area index as inputs. Additional estimating the wind tunnel test facility, and the resources to test a equations were developed to predict plant area or plant large number of species are not available. mass from simple geometric measurements of yucca and The first objective of this study was to begin sagebrush. Finally, for shrub or stubble canopies in developing (SG)e prediction equations for shrub which (SG)e prediction parameters have not been canopies. Two species were selected for initial study measured, a way to approximate the prediction because they are widespread and have contrasting plant parameters using an estimate of canopy aerodynamic structures. These species were sand sagebrush roughness length was developed. {Artemisia filifolia Nutt.) and yucca {Yucca elata Engelm.). A second objective of the study was to develop INTRODUCTION a methodology to permit estimation of (SG)e for shrubs that had not been tested in the wind tunnel. Large areas in the U.S. are covered with shrub- dominated rangelands. Among the most important THEORY shrub-dominated rangeland ecosystems where wind erosion can be a problem are the following: sagebrush, The small grain reference standard (SG)^ has been pinyon-juniper, creosote-tarbush, mesquite, and defined as 0.254 m (10 in.) long, dry, small grain (wheat) shinnery oak.* Nutrients are often concentrated near the stalks lying flat on the soil surface in rows perpendicular surface in rangeland soils, and soil trapped in wind to wind direction with 0.254 m (10 in.) row spacing and erosion catchers on rangelands on the average has higher stalks oriented parallel to the wind direction. nutrient enrichment ratios than soil eroding from Experimental data relating (SG)^ to plant biomass can croplands (Hagen and Lyles, 1985). generally be closely fitted by an empirical prediction Because rangeland productivity is already often equation of the form limited by soil water-holding capacity and/or nutrient availability, it is important to consider wind erosion (SG)e = a Rb [1] control in design and evaluation of rangeland where R^ is the air dry mass of aboveground vegetation cover and a, b are the prediction equation parameters whose value depends on the kind of vegetation cover (Lyles and Allison, 1981). Article was submitted for publication in December, 1987; reviewed We can also expresss R^ as and approved for publication by the Soil and Water Division of ASAE in March, 1988. Contribution from the U.S. Department of Agriculture, Agricultural R„ [2] Research Service, in cooperation with Kansas Agricultural Experiment Station, Contribution Number 88-135J. The authors are: L. J. HAGEN, Agricultural Engineer, and LEON where LYLES, Research Leader, USDA-ARS, Kansas State University, Manhattan, KS. N/A^ number of plants per unit area •Personal communication from Don Pendlton, Soil Conservation A. silhouette area (projected area facing the Service, National Range Scientist. flow) Vol. 31(3):May-June, 1988 769 FREESTRCAM VELOCITY Finally, Shaw and Pereira (1982) noted that it is really the product of the drag coefficient (C^) and the plant area that depletes the momentum flux through the VEGETATION LOG-LAW LAYER canopy. Hence, PAI should be replaced by C^ • PAI in equations [3] and [6]. However, for the stalks, branches, <^v-^v'^> and stiff yucca leaves discussed in this work, €^=1.0. In canopies with a significant number of leaves that streamline with the air flow under erosive wind speeds, it appears necessary to weight C^ to reflect the VEGETATIVE proportionate areas of leaves and stalks. Shaw and CANOPY \ I 1/ y N N1 / N K T Pereira (1982) used C^ = 0.2 for a growing corn canopy. ( C^PAI) V V When measuring (SO^ in a wind tunnel, the V k freestream velocity (Uoo) is held constant. U^^ will then LO"6'^L'A¥nu7oT^r vary only in response to canopy surface roughness (Z^v), LAYER (o.u;* 2;>i rf-ffiffff JSL2±&. t ff g gj which can be expressed in the form Fig. 1—Schematic representatioii of wind erosion and some of the U. = C. ZP [7] important variables in a standing vegetative canopy. V O OV where C^, and p are constants over a moderate range of p = plant density Z„ (Lyles and Allison, 1979). Combining equations [3] f(d) = some function of plant thickness parallel to through [7] gives the flow For example, with standing stalks, A^ = height times (SG)e = C(Cd-PAI)b = Cj •Czln diameter and f(d) = TI—, where d is stalk diameter. The 4 term (N A/Aj) is often referred to as the plant area 0 OV index (PAI). Using PAI equation [1] can be rewritten as C4+C'5(Cd-PAI) ]| .[8] (SG)g = C(PAI)b [3] Equation [8] illustrtes that the (SG)e should only be a where C is a new parameter. function of C^ • PAI and Z^^. Further, at any fixed The (SG)e can also be expressed as the rate of sand Cj • PAI, variation in the prediction parameters, C and flux {q(kg/m-s)} below a vegetative canopy. An empirical b, of (SG)e should be caused by differences in Z^v among fit of data from Lyles and Allison (1980) shows that for canopies. (SGX > 200 kg/ha EXPERIMENTAL PROCEDURE (SG), = Ci-C2ln(q) [4] With the help of the USD A, Soil Conservation Service, two range sites located in Stevens County in southwestern where Cj and C2 are constants. Fryrear (1985) has Kansas were selected for experimental study. The sites summarized wind erosion soil loss data from a variety of were relatively level and had nearly pure stands of either experiments and found a similar expression that fit the sand sagebrush or yucca with sparse, interspersed grass data. 0.01 to 0.05 m tall. Average height of sagebrush was 0.68 Wind erosion in a standing vegetative canopy is m and of yucca was 0.79 m. On each site, a triangularly illustrated in Fig. 1, where q below the canopy is a shaped plot (base 116 m and height 99 m) was staked function of surface saltation friction velocity (UIQ). with the base oriented normal to the prevailing southerly Assuming that Bagnold's formulation (Greeley and wind direction. Instrument towers were placed windward Iverson, 1985) holds below a canopy, then (south) of each plot and leeward in the apex of the triangle at the north end of each plot. Sensitive cup c^ u;3 [5] anemometers, thermocouples, and a direction indicator were mounted on the windward tower. When the where C3 is a constant. Other wind tunnel measurements direction vane on the windward tower indicated southerly (Lyles and Allison, 1976) have shown that the winds and the temperature profile indicated near neutral relationship of friction velocity above the canopy (U^^) to stability, the aerodynamic properties of the plot were that below without saltation (U^Q) can be expressed as measured at the leeward towers. Two towers were used at the lee position. Sensitive cup U* :C4 + C5(PAI) [6] anemometers were located on a portable tower at heights u*. of 0.245, 0.508, and 0.762 m, and a small thermal velocity probe was located 0.005 m above the surface. To Using a numerical model of a complex plant canopy, measure the average airflow in the canopy, the portable Shaw and Pereira (1982) obtained a similar result. They tower was moved to 10 positions along a line normal to also found that C5 varied as the soil surface roughness the wind direction in 1 m increments. Wind speeds at below the canopy varied. Hence, with saltation, we will each position were averaged for 5 min. In addition, wind denote the constants as C4 and Q in equation [6]. speeds above the leeward canopy were also measured by 770 TRANSACTIONS of the ASAE cup anemometers on a fixed tower at 1.0, 1.5, 2.5, and 1.0 4.0 m above the surface. -•-SAGEBRUSH Each main plot was divided into 90 subplots (5.5 X — A- YUCCA 12.0 m) by wire stake markers.