Catchment asymmetry is a fairly frequent phenomenon on a global scale but the main causes leading to its formation are still not well understood. Where the intervention of structural or tectonic causes is not relevant, asymmetry seems to result from differential erosion between opposite slopes that flow into the same channel, which is frequently associated with contrasted biocrust and/or vegetation covers. Biocrusts are known to be important surface stabilizing agents. However, their geomorphological consequences at the landscape scale are little known. In this study we combined field measurements with digital elevation models and image analysis to determine whether catchment asymmetry in the Tabernas Desert (semi-arid SE of Spain) is a local or general phenomenon, and to explore the main factors determining asymmetry occurrence, magnitude and direction. We pay special attention to the role of biocrusts. We found that catchment asymmetry is a very common phenomenon in the area; only 25% of the catchments are symmetrical, while approximately 40% present asymmetry with the relatively shady hillslope having a lower gradient, and 35% with that hillslope being the steeper. Solar radiation reaching the soil, surface area and channel gradient in the considered catchment stretch, as well as the total catchment area upstream from the lower point of the considered stretch were the main abiotic factors controlling the formation of the asymmetry. Microclimatic differentiation due to differences in radiation input caused by the uneven topography favoured the relative stabilization of the shadier hillslope and its colonization by biocrusts and later by plants. The effect of the biocrusts and vegetation protection against water erosion on shadier hillslopes is often stronger than that of the set of abiotic factors and gives rise to asymmetries with lower gradients in the shady hillslope by promoting lateral displacement of the channel. We hypothesised that the opposite pattern, with the sunnier hillslope having a lower gradient, occurs when abiotic factors control the development of asymmetry formation. In these conditions, the effect of biocrusts and plants would act in the opposite direction. We propose a conceptual model of feedbacks generating catchment asymmetry, with biocrust playing a crucial role.

集水区不对称在全球范围内是一种相当常见的现象，但导致其形成的主要原因仍不清楚。在结构或构造原因的干预不相关的情况下，不对称似乎是由于流入同一通道的相对斜坡之间的差异侵蚀造成的，这通常与对比鲜明的生物地壳和/或植被覆盖有关。已知生物结皮是重要的表面稳定剂。然而，它们在景观尺度上的地貌后果鲜为人知。在这项研究中，我们将实地测量与数字高程模型和图像分析相结合，以确定 Tabernas 沙漠（西班牙半干旱东南部）的集水区不对称是局部现象还是普遍现象，并探讨决定不对称发生的主要因素，大小和方向。我们特别关注生物结皮的作用。我们发现流域不对称是该地区非常普遍的现象；只有 25% 的集水区是对称的，而大约 40% 的集水区不对称，相对阴凉的山坡坡度较低，35% 的坡坡较陡。到达土壤的太阳辐射、所考虑的集水区的表面积和通道梯度，以及所考虑的流域较低点上游的总集水区面积是控制不对称形成的主要非生物因素。由于地形不均匀引起的辐射输入差异导致的小气候分化有利于阴凉山坡的相对稳定及其生物结皮和后来植物的定植。生物地壳和植被对阴凉山坡的水蚀保护作用通常强于一组非生物因素的作用，并通过促进河道的横向位移，在阴凉的山坡上导致坡度较低的不对称性。我们假设当非生物因素控制不对称形成的发展时，会出现相反的模式，即阳光充足的山坡具有较低的坡度。在这些条件下，生物结皮和植物的作用将相反。我们提出了一个产生集水不对称的反馈概念模型，其中生物地壳起着至关重要的作用。