Relative scales of time and effectiveness of climate in watershed geomorphology
Relative scales of time and effectiveness of climate in watershed geomorphology
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DOI：
10.1002/esp.3290030207
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发表时间：
1978-04
影响因子：
3.3
通讯作者：
M. Wolman;R. Gerson
中科院分区：
文献类型：
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作者：
M. Wolman;R. Gerson
在世界范围内的气候范围内，单位面积的峰值降雨量和峰值径流量是可比的。然而，虽然不同地区的能量或力量的外部贡献的大小相似，但对景观的影响在不同地区之间明显不同。气候事件的绝对震级和这些事件之间的绝对时间间隔不能令人满意地衡量不同震级和重现间隔的事件的地貌有效性。虽然地貌过程是由一系列相互关联的气候、地形、岩性和生物因素驱动的，但个别极端事件所做的工作可以用平均年侵蚀的比率来衡量，这些事件在形成景观特征方面的有效性可能与极端事件改变后河道形态的恢复速度或大量浪费的伤痕有关。因此，有效性的时间尺度可以将事件的重现间隔与地貌恢复事件之前存在的形态所需的时间联系起来。在温带地区，洪水每隔50年至200多年反复发生，拓宽的河道可能在几个月或几年内恢复原来的宽度。在半干旱地区，河道形态的恢复不仅取决于流量，还取决于滩地植被生长的气候决定因素，从而产生不同的恢复率，这取决于平均流量和强化植被的重合程度。在真正干旱的地区，缺乏植被和水流阻碍了恢复，在流域地区，渠道宽度增加到100平方公里，但在较大的流域地区，渠道宽度保持相对不变。面积和时间控制特定事件的有效性，因为同时出现峰值放电或降雨和大面积的可能性较小，特别是在干旱地区，那里发生的面积超过数千平方公里的事件如果发生的话极其罕见。在某种程度上，湿润地区面积的减少可以与区域从湿润气候向更干旱气候的变化相媲美，反映在与更连续的过程相比，间歇性气候的重要性增加。极罕见的极端规模的洪水，估计重复发生的时间间隔为500年或更长时间，可能会超过“正常”记录中无法达到的能力门槛，导致山谷地貌的“不可挽回”的变化。在相对较少的事件中，由于大量浪费造成的山坡剥蚀也可能与平均剥蚀率有关，也与不同类型的山体滑坡留下伤痕后山坡表面的恢复有关。文献中描述的测量恢复时间从一些热带地区的不到十年到温带地区的几十年或更长。在一些热带地区，引发大规模浪费的高强度风暴的重复间隔从一到两年不等，在一些季节性降雨区从每百年3到4年，在一些温带地区到100年或更长时间。气候事件在山坡和河流上的有效性与坡度、岩性或其他控制活动阈值和恢复速度的变量是分不开的。
Peak rainfalls and peak runoff rates per unit area are comparable over a worldwide spectrum of climates. However, while the magnitude of the external contribution of energy or force in diverse regions is similar, the impact on the landscape varies markedly between regions. Absolute magnitudes of climatic events and absolute time intervals between such events do not provide satisfactory measures of the geomorphic effectiveness of events of different magnitudes and recurrence intervals. Although geomorphic processes are driven by complex sets of interrelated climatic, topographic, lithologic, and biologic factors, the work done by individual extreme events can be scaled as a ratio to mean annual erosion and the effectiveness of such events in forming landscape features can be related to the rate of recovery of channel form or mass wasting scars following alteration by the extreme event. Thus, a time scale for effectiveness may relate the recurrence interval of an event to the time required for a landform to recover the form existing prior to the event. River channels in temperate regions widened by floods of recurrence intervals from 50 to more than 200 years may regain their original width in matters of months or years. In semi-arid regions, recovery of channel form depends not only upon flows but upon climatic determinants of the growth of bottomland vegetation resulting in variable rates of recovery, on the order of decades, depending upon coincidence of average flows and strengthened vegetation. In truly arid regions the absence of vegetation and flow precludes recovery and the width of channels increases in drainage areas up to 100 km2 but remains relatively constant at larger drainage areas. Area as well as time controls the effectiveness of specific events inasmuch as the likelihood of simultaneous peak discharges or rainfalls and large areas is less, particularly in arid regions where events spanning areas of more than several thousand km2 are extremely rare if experienced at all. To some extent a decrease in area in a humid region is comparable with a regional change from humid toward more arid climate reflected in the increase in importance of episodic as contrasted with more continuous processes. Exceedingly rare floods of extreme magnitudes, estimated recurrence intervals of 500 years or longer, may exceed thresholds of competence otherwise unattainable in the ‘normal’ record resulting in ‘irreparable’ transformations of valley landforms. Denudation of hillslopes by mass wasting during relatively rare events can also be related to mean rates of denudation and to recovery of hillslope surfaces after scarring by different kinds of landslides. Measured recovery times described in the literature vary from less than a decade for some tropical regions to decades or more in temperate regions. Recurrence intervals of high magnitude storms which trigger mass wasting range from 1 to 2 years in some tropical areas, to 3 or 4 per hundred years in some areas of seasonal rainfall and to 100 or more years in some temperate regions. The effectiveness of climatic events on both hillslopes and rivers is not separable from gradient, lithology or other variables which control both thresholds of activity and recovery rate.