Influence of silvicultural practices on the hydrology of pine flatwoods in Florida

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Slash pine (Pinus elliottii Engelm.) flatwoods in diked watersheds were harvested and regenerated with high and low levels of disturbance. One watershed received machine harvesting, slash burning, windrowing, bedding, and machine planting, which are practices causing a high level of disturbance. A second watershed received manual harvesting, slash chopping, bedding, and machine planting, which are practices causing a low level of disturbance. Water regimes of the two watersheds were compared against that of the third, an undisturbed control. Water table levels were higher after harvest because of reduced evapotranspiration (Et) posttreatment. The differences in water table levels due to treatment were more pronounced at deeper soil depths, probably owing to a lower specific yield of the soil matrix with depth. Daily runoff during the first year increased somewhat in proportion to the area clear‐cut in each watershed. However, only the 15‐cm first‐year runoff increase from the high‐disturbance watershed was significant at 150% of predicted runoff. This increase dropped to 65% of predicted runoff 6 years later, suggesting a return to normal about 11 years after treatment. In contrast, an unexpectedly early decrease in runoff from the low‐disturbance watershed was apparent by the last year indicative of anomalous watershed behavior likely due to greater seepage through the perimeter dikes. A severe drought which occurred during the study caused a very slow recovery of the hydrologie regime; it took a full year of normal rainfall to restore runoff patterns. Moreover, the drought demonstrated the importance of pond storage in the attenuation of surface runoff. High‐disturbance site preparation resulted in many windrows oriented toward the drainages, less resistance to surface runoff, and some compaction of normally wet soils. The net effect was less infiltration and more storm flow. First‐year storm flow was increased on the high‐disturbance watershed, but flooding by large storms often overwhelmed this effect. Pine Et estimates were used in the annual water balances to calculate seepage. A seepage value of 3 cm/yr, or 2% of annual rainfall, showed good agreement with previously published information for pine flatwoods.