Water-Level Fluctuation in Wetlands As a Function of Landscape Condition in the Prairie Pothole Region
Results
Our analysis of water-level fluctuations, corrected for catchment size (Table 1), identified water-level fluctuation as a function of wetland condition (Figure 5). We observed both a wetland condition effect (F = 7.08, 1,21 df, P = 0.0146) and a wetland class effect (F = 4.88, 2,21 df, P = 0.0182). Further, there was no condition-by-class interaction (F = 0.86, 2,21 df, P = 0.4376) to complicate the interpretation. In proportion to catchment size, wetlands in agricultural landscapes had greater water-level fluctuations (14.14 cm, UCL = 19.73, LCL = 10.05) than wetlands in landscapes dominated by grassland (4.27 cm, UCL = 5.71, LCL = 3.14). As expected, due to their greater reliance on surface runoff as a water source, seasonal wetlands and temporary wetlands had greater water level fluctuations (11.82 cm and 13.74 cm, respectively) than semipermanent wetlands (2.77 cm, P = 0.0220 and P = 0.0090, respectively). We found no difference in water-level fluctuation between seasonal and temporary wetlands (P = 0.7775).
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| Figure 5. Mean (black-transformed LSM ± approximate standard error) 1993 water-level fluctuations of temporary, seasonal, and semipermanent wetlands located in areas dominated by agricultural lands and grasslands. |
Table 1. Water-depth fluctuations (corrected for catchment size) of temporary, seasonal, and semipermanent wetlands from grassland and cropland dominated landscapes in the Prairie Pothole Region of North and South Dakota, 1993.
| Water depth (cm) | Catchment | Corrected | |||||
| Plot | Wetland | Landscape | Class | Maximum | Minimum | Size(ha) | Fluctuation |
| 73 | 86 | Grass | Temp | gauge destroyed by cattle | |||
| 156 | 26 | Grass | Temp | 11.0 | 0.0 | 2.6 | 4.2 |
| 363 | 58 | Grass | Temp | 34.0 | 18.0 | 6.8 | 2.4 |
| 374 | 65 | Grass | Temp | 16.5 | 0.0 | 9.9 | 1.7 |
| 407 | 109 | Grass | Temp | 31.0 | 0.0 | 0.9 | 34.4 |
| 498 | 227 | Grass | Temp | 17.0 | 0.0 | 3.3 | 5.2 |
| 156 | 24 | Grass | Seas | contained no water in 1993 | |||
| 156 | 42 | Grass | Seas | 71.0 | 0.0 | 5.9 | 12.0 |
| 374 | 225 | Grass | Seas | 76.0 | 62.0 | 5.5 | 2.5 |
| 374 | 272 | Grass | Seas | 77.0 | 74.0 | 2.0 | 1.5 |
| 407 | 67 | Grass | Seas | gauge destroyed by cattle | |||
| 498 | 277 | Grass | Seas | 37.0 | 18.0 | 5.8 | 3.3 |
| 73 | 29 | Grass | Semi | 75.0 | 22.0 | 26.1 | 2.0 |
| 156 | 22 | Grass | Semi | gauge destroyed by cattle | |||
| 363 | 22 | Grass | Semi | 56.0 | 50.5 | 5.6 | 1.0 |
| 374 | 100 | Grass | Semi | 81.0 | 77.5 | 5.7 | 0.6 |
| 407 | 168 | Grass | Semi | 44.0 | 0.0 | 170.5 | 0.3 |
| 498 | 146 | Grass | Semi | 50.0 | 26.0 | 9.8 | 2.4 |
| 133 | 370 | Crop | Temp | contained no water in 1993 | |||
| 134 | 270 | Crop | Temp | 26.0 | 0.0 | 0.8 | 32.5 |
| 134 | 432 | Crop | Temp | contained no water in 1993 | |||
| 327 | 72 | Crop | Temp | 34.0 | 0.0 | 2.2 | 15.5 |
| 442 | 260 | Crop | Temp | 34.0 | 0.0 | 0.5 | 68.0 |
| 442 | 261 | Crop | Temp | 39.0 | 0.0 | 1.0 | 39.0 |
| 133 | 386 | Crop | Seas | contained no water in 1993 | |||
| 134 | 158 | Crop | Seas | 21.5 | 0.0 | 1.2 | 17.9 |
| 134 | 406 | Crop | Seas | 50.5 | 20.0 | 2.7 | 11.3 |
| 327 | 147 | Crop | Seas | gauge malfunctioned | |||
| 442 | 93 | Crop | Seas | 68.0 | 0.0 | 0.3 | 226.7 |
| 442 | 281 | Crop | Seas | 85.0 | 0.0 | 5.2 | 16.3 |
| 133 | 380 | Crop | Semi | contained no water in 1993 | |||
| 134 | 140 | Crop | Semi | 40.5 | 31.0 | 1.0 | 9.5 |
| 134 | 165 | Crop | Semi | 27.5 | 0.0 | 5.4 | 5.1 |
| 327 | 117 | Crop | Semi | 88.0 | 39.0 | 15.6 | 3.1 |
| 442 | 295 | Crop | Semi | 41.5 | 0.0 | 46.5 | 0.9 |
| 442 | 301 | Crop | Semi | 47.5 | 0.0 | 18.8 | 2.5 |
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| Figure 6. Water levels of wetlands P7 and P8, Cottonwood Lake Study Area, Stutsman County, ND as recorded by Telog water-level monitoring systems. |
As determined by the Telog water-level monitoring systems installed at CLSA, water levels of wetland P7 (Figure 6a) reached a maximum depth of 28.2 cm on June 6 in 1992, after which levels steadily fell until the wetland went dry on July 23. In 1993, due to record high rainfall, the trends were reversed, with wetland P7 steadily gaining water through most of the summer. In 1993, depths of wetland P7 were minimum (39.6 cm) on April 5 and maximum (131.3 cm) on August 31. Water levels of wetland P8 (Figure 6b) followed the same general trends, reaching a maximum depth of 25.9 cm on July 2 in 1992 and steadily falling to dryness on August 18. Wetland P8 gained water throughout most of the summer of 1993, increasing from a minimum depth of 63.6 cm on May 6 to a maximum of 136.7 cm on July 25.
Table 2. Maximum and minimun water levels (cm) of wetland P7 and P8 at the Cottonwood Lake Study Area, Stutsman County, North Dakota, as recorded by prototype water-level recorders.
| 1992 | 1993 | |||
| Wetland | Maximum | Minimum | Maximum | Minimum |
| P7 | 27.9 | 0.0 | * | 39.4 |
| P8 | 25.4 | 0.0 | * | 65.0 |
Throughout the study period, our prototype water-level recorders recorded the maximum and minimum water levels of wetlands P7 and P8 at CLSA, providing data within 1.4 cm of the readings provided by the Telog water-level monitoring systems. The only exception was in 1993 when record rainfall caused water levels to exceed the capacity of our recorders (Table2); the rods on the prototype devices were too short to record the increases. A frigid winter (lows to -40°C) did not damage our devices, but we noted that the copper-coated steel guide rods were corroded. Their copper coatings had either been scratched or were plated too thinly. We replaced the guide rods when the devices were checked in the spring of 1993 so the corrosion would not affect readings for this study.
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