40474 Federal Register / Vol. 56, No. 157 / Wednesday, August 14, 1991 / Proposed Rules 



Chenopodium nibrum, and Senecio 

 conges tus. 



For Cropland Areas 



Cropland Drawdown Phase 



Tilled pothole bottoms with 

 drawdown vegetation characterize this 

 phase. The plants include many coarse, 

 introduced annual weeds and grasses 

 that normally develop on exposed mud 

 flats during the growing season. These 

 species appear as overv^ter emergents 

 whenever surface water is restored by 

 summer rains. Characteristic plant 

 species include: Agropyron repens, 

 Echinochloa ctvsgalli. Polygonum 

 lapathifolium, Veronica peregrina, 

 Hordeum jubatum, Plagiobothrys 

 scopulorum, Xanthium italicum, Bidens 

 frondosa, Seteria glauca. Polygonum 

 convolvulus, Agropyron smithii, 

 Brassica kaber, Descruainia sophia, 

 Androsace occidentalis, Ellisia 

 nyctelea, Erigeron canadensis, and Iva 

 xanthifolia. 



Cropland Tillage Phase 



In this phase, tilled bottom soils are 

 dominated by annual field weeds, 

 characteristic of fallow or neglected low 

 cropland. Tilled dry pothole bottoms 

 devoid of vegetation are also considered 

 to be in this phase. Planted small grain 

 or row crops are often present. 



Playas 



Playas occur in many arid or semiarid 

 regions of the world. Although occiuring 

 throughout much of the western United 

 States, they are concentrated in the 

 southern Great Plains as either 

 ephemeral or permanent lakes or 

 wetlands (Nelson et. al. 1983). The 

 topography of most playa regions is flat 

 to genUy rolling and generally devoid of 

 drainage. Runoff from the surrounding 

 terrain is collected into playa basins, 

 where water is evaporated rapidly. 

 Playas range in size from several 

 hundred acres to only a few acres, with 

 the majority being less than 10 acres. 



Surface soils of playas are generally 

 clays that form a highly impermeable 

 seal and increase their water-holding 

 capacity. The playa soUs are typically 

 VerUsols. In the southern Great Plains, 

 playa soils are listed as Randall, Lipan, 

 or Ness days, Stegall silty clay loams, 

 Lofton clay loams, or may be 

 uncharacterized occurring as inclusions 

 within nonhydric soil map units. Soils of 

 playas are generally distinguishable 

 from surrounding upland soils because 

 of their contrasting darker color (Reed 

 1930). 



The hydrology of playas involves 

 rapid accumulation of natural nmoff 

 during late spring, with a gradual loss by 



evaporation and seepage through the 

 summer except where basins have been 

 excavated to concentrate water. The 

 hydrology is influenced by agricvdtural 

 practices, including basin modification 

 for water collection and retention and 

 grazing in the watershed. Water 

 reaching the playa is derived primarily 

 from precipitation and runoff within the 

 basin watershed. 



Playa basins are dry most of the time. 

 The basins collect water primarily in 

 two peak periods — May and 

 September — as a result of regional 

 convectional storms conunon throughout 

 the region. Water collection in the 

 basins is generally representative of 

 seasonal or long-term extremes and not 

 average annual conditions. As a result, 

 wetland hydrology is best characterized 

 by examining hydrological indicators 

 over a multi-year period rather than 

 relying on hydrological conditions that 

 may be present at any point in time. 



The hydrology of most playa wetlands 

 seldom allows a stable flora to develop. 

 Playa basins may have a dense cover of 

 annual or perennial vegetation or may 

 be barren, depending on the timing, 

 intensity and amount of precipitation 

 and irrigation nmoff, the extent of 

 grazing, and the size of the playas. As 

 with potholes, the process of annual 

 drying (drawdown) in playas enables 

 the invasion of FAC, FACU, and UPL 

 plants during dry periods which may 

 persist into other seasons. Playa basins 

 may show vegetative zonation in 

 concentric bands from the basin center 

 to the perimeter in response to 

 decreasing water depths or soil moisture 

 levels. However, such zonation is not 

 typical of all playa basins; small playas 

 that collect limited runoff may support 

 prairie vegetation (primarily FACU and 

 UPL species) or may be cultivated. 

 Cultivated basins often contain either 

 the Uving plants or remnants of 

 smartweeds [Polygonum spp.), ragweeds 

 [Ambrosia spp.), or other invading 

 annuals. Some playa basins are large 

 enough to have an open expanse of deep 

 water that may support aquatic plant 

 communities. 



Vernal Pools 



Vernal pools are depressional areas 

 covered by shallow water for variable 

 periods from winter to spring, but may 

 be completely dry for most of the 

 summer and fall. Small pools may drain 

 completely several times during the 

 rainy season eind some pools may not 

 retain any water during drought years. 



An understanding of the natural 

 history of the plants that occur in the 

 transitional areas from pool to typically 

 terrestrial habitat is useful in delineating 

 these wetiands. Zedler (1987) provides 



an excellent overview of vernal pools 

 which is briefly summarized below. 



Vernal pools are wride-ranging in size 

 (from 10 feet wide to 10 acres) but are 

 always shallow (less than 6 inches to 2 

 feet deep). Depth and duration of 

 saturation and inundation are more 

 important in defining a vernal pool than 

 size. Soils with confining layers, either 

 nearly impermeable clay layers or iron- 

 silica cemented hardpans, often have a 

 seasonally perched water table which 

 favors the development of vernal pooh 

 MicroreUef on the soils typically is 

 hummocky, writh pits (depressions) and 

 mounds. Individual vernal pools are 

 often interconnected by a series of 

 swales and tributaries. Winter rainfall 

 perches on the confining layer, until 

 removed by evapotranspiration in the 

 spring. A cemented hardpan or nearly 

 impermeable clay subsoil layer, the pit 

 and mound microrelief, and presence of 

 swales are strong indicators of vernal 

 pools. 



Vernal pools hold water long enough 

 to allow some strictiy aquatic organisms 

 to grow and reproduce (complete their 

 life cycles), but not long enough to 

 permit the development of a typical 

 pond or marsh ecosystem. Changes in a 

 vernal pool during the season are so 

 dramatic that it is in some ways more 

 appropriate to consider it to be 

 sequence of ecosystem (a cyclical 

 wetland) rather than a single static type. 

 Vernal pool development can be broken 

 into four phases: (1) Wetting phase, (2) 

 aquatic phase, (3) drying phase, and (4) 

 drought phase. The first rains stimulate 

 the germination of dormant seeds and 

 the growth of perennial plants (wetting 

 phase). When the cumulative rainfall is 

 sufficient to saturate the soils, aquatic 

 plants and animals proliferate (aquatic 

 phase). Nonaquatic plants are subjected 

 to stress at this time. As the pool levels 

 begin to recede (drying phase), the high 

 soil moisture insures that plant growth 

 continues after standing water is gone. 

 Eventually, the plants succumb to 

 drought and turn browm, with drying 

 cracks appearing in the soil (drought 

 phase). 



Plant species characteristic of vernal 

 pools are endemic to vernal pools, or 

 occur in vernal pools but are common in 

 other aquatic habitats or associated 

 with vernal pools (see Tables 6A-D in 

 Zedler, 1987). Non-pool species can 

 tolerate the limited periods of standing 

 water that exist toward the pool 

 margins. 



Since vernal pools typically vary 

 considerably in depth and duration or 

 both from year to year, within a year, or 

 between different pools, plant 

 composition is quite dynamic. FAC, 



