V/ t. Wetland Hydrology NREM 665

Transcription

1 V/ t Wetland Hydrology NREM 665

2 I. Hydrology = most critical factor for establishment & maintenance of specific WTL types & processes (M&G p. 108) A. Hydroperiod: seasonal pattern of H 2 O level of a wetland; 1. The hydrologic signature of a WTL 2. Some generalized hydroperiods for a diverse set of WTLs 2

3 3

4 3. Recurring hydroperiod patterns: a. pulsing in tidal & riparian tidal: diurnal & seasonal patterns b. stable, consistent hydroperiods in GW-fed WTLs c. seasonal fluctuation: high WT in wint/sp, drop in sum w/ ET 4

5 II. Water Budget Equation V/ t = P + Si + Gi ET So Go ± T V/ t = change in volume per unit time, t V = volume of H 2 0 stored in WTL P = precipitation Si = surface inflow Gi = groundwater inflow ET = evapotranspiration So = surface outflow T = tidal inflow (+), outflow (-) V/ t A. Example H 2 O budgets 5

6 (Mitsch & Gosselink 2000) 6

7 III. P A. Input to all WTL types B. WTLs occur where P > ET & SO C. Measurement: onsite rain gauge or local weather station 7

8 Precipitation P n = SF + TF n Where SF = stemfall TF = throughfall Interception I = P (SF + TF) (Mitsch & Gosselink 1993) 8

9 IV. Si & So A. Doesn t affect all WTLtypes 1. Most imp. in which types? B. WTLs subject to different types of Sis 1. overland flow (non-channelized sheet flow from upland to WTL) 2. stream flow, overbank flooding (stream to WTL) 9

10 C. Measurement: Si, So = A x * V A x = stream cross sectional area (units = dist 2 ) V = average velocity (units = dist/time) D. Rating curve: plot of velocity vs stage Vel/discha arge (diff) Stage/elev (easy) 10

11 E. Manning equation: estimate of surface flow with no available velocity measurements S = i ( o) A x R 2/3 n s 1/ 2 S = surface flow A x = cross-sectional area of water course R = hydraulic radius (m) s = channel slope n = Manning roughness coefficient Hydraulic radius: Cross-sectional area divided by wetted perimeter 11

12 (Mitsch & Gosselink 1993) 12

13 Manning s n associated with wetland vegetation Vegetation Type n Citation Schoenoplectus tabernaemontani (800 stems/m 2 ) Hall & Freeman 1994 Cladium mariscus Lee & Carter (undated) Heavy timber USFS 13

14 V. GWi & GWo A. major H 2 O source for some types B. know less about GW than P, S, T C. discharge, recharge, throughflow 14

15 D. Measurement: Darcy s Law G = k A x s G = GW flow rate (volume/time, m 3 /day) k = hydraulic conductivity (dist/time, m/day) A x = GW cross sectional area (area, m 2 ) S = hydraulic gradient (unitless, slope of piezometric surface) 15

16 (Mitsch & Gosselink 1993) 16

17 E. GW measurements w/ wells & piezometers 1. Piezometer (cased well): a tube that is slotted at a particular depth used to measure piezometric (hydraulic) head at the depths of the slots 2. Groundwater well (observation well, perforated pipe): a tube that is slotted from top to bottom. Wells are used to measure the depth to the water table. (WRP 1993) 17

18 (WRP 1993) groundwater well installation piezometer installation 18

19 3. Piezometer terminology a. Pressure head: height of water column in the piezometer b. Gravitational head: distance above or below a standard elevation (in this class we will consider soil surface to be the datum) c. Piezometric head: sum of pressure head & gravitational head 19

20 4. How to use piezometers to measure direction of GW flow? a. Groundwater flows from high to low piezometric head b. Measuring groundwater level in a series of nested piezometers can indicate discharge/recharge activity 20

21 P1 P2 P3 Pres head Grav head Piez head P1 P2 P3 (WRP 1993) 21

22 P1 P2 P3 Pres head Grav head Piez head P1 P2 P3 (WRP 1993) 22

23 VII. ET A. All WTL types affected by ET 1. evap: moisture that vaporizes from soil or open H transpiration: moisture that passes from vascular plants to atmo. B. Direct measurements of ET 1. pan evap.: wt. or vol. of H 2 O lost from pan over time a. measure of potential ET b. simple, cheap; unrealistic 2. diurnal method: det. ET from heat & H 2 O balance thru plant canopy a. must have uniform veg, H 2 O table near root zone 23

24 Evaporation at o pan evaporation_substation.htm 24

25 C. Empirical estimates 1. Thornthwaite Equation for potential ET PET m = 16(10T m /I) b = PET (mm/month) T m = monthly mean temp ( o C) 12 I = local heat index = (T m /5) m = 1 b = (0.675I 3-771I 2 +17,920I+492,390) 920I+492,390)*10-6 (slightly diff than PS1) a. Simple; doesn t work below freezing; doesn t account for advection 25

26 2. Penman equations based on Dalton s law & energy budget approach ET = H E a = slope of saturation ti vapor pressure vs mean air temp (mm Hg C -1 ) H = net radiation = R t (1-a) - R b (cal cm -2 day -1 ) R t = total short wave radiation a = albedo of wetland surface R b = effective outgoing longwave rad = ƒ(t 4 ) E a = 0.35( u)(e w -e a ) u = wind speed 2 m above ground (km/day) e w = saturation vapor pres. of H 2 O mean air temp (mmhg) e a = vapor pressure of surrounding air (mmhg) a. requires numerous measurements, expensive eqpt. 26

27 VII. Tides A. Don t effect all WTL types B. Subsidy-Stress concept Tidal flushing = STRESSOR, causes submergence, high salinity, anaerobic soils Tidal flushing = SUBSIDY, removes excess salts, reestablishes aerobic soils, provides nutrients 27

28 C. Measurement: H 2 O level recorders, clod card dissolution (Thompson & Glenn 1994) 28

29 (Mitsch & Gosselink 1993) 29

30 VIII. Effects of hydrology on structure & function A. Species composition & richness 1. Hydrology selects H 2 O tolerant veg, excludes intolerant spp a. relatively few spp adapted for WTL conditions 2. Flowing H 2 O stimulates diversity; const. flooding/drought don t B. Primary productivity 1. Wetland subject to pulsing have higher prod. than wetlands in stagnant, flooded conditions Type of Cypress Swamp NPP (g) Stagnant 192 Cypress dome 600 Very slow flowing water 692 Riverine undrained edge stand 1170 Semiriverine, seasonal flooding 1140 (Conner & Day 1976) 30