**1. Introduction**

The mining industry plays a crucial role in global economic development, but it often faces environmental challenges, particularly related to rehabilitated landform design and its erosional stability. Constant efforts must be made to develop and build an appropriate landform and cover system that effectively serves its intended purposes, such as supporting vegetation growth and preventing the infiltration of water and oxygen into reactive mine waste. The success of the soil cover system depends on the external surface of the landform remaining intact. If the surface erodes, the cover system's functions are compromised, and it is likely to fail. To evaluate the long-term erosion stability of a constructed landform, erosion rate thresholds must be established to ensure the landform is acceptably resistant to erosion, or "stable."

While the defensible erosion rate thresholds should come close to the term "tolerable or acceptable erosion" rate which was first proposed by Browning et al. [1], and a more thorough review of tolerable values was carried out by the Soil Science Society of America in 1979; there is still no broad agreement on what we could consider as a tolerable or "acceptable" rate of erosion on a rehabilitated landform.

In general, US soil conservationists have consistently based tolerable soil loss values largely on the natural soil forming rate and on consideration of the maintenance of soil productivity, although accepting that other factors may be important in some situations. The US soil conservation agencies have typically used tolerable erosion values of < 11.2 t/ha/y for deep fertile soils, and < 4.5 t/ha/y for shallow agricultural soils. However, the fact that these values have been quantified under the American agricultural soil conditions must be kept in mind; that makes it doubtful to blindly accept these values to judge or establish defensible erosion rate thresholds under the Australian mining conditions. Therefore, the Queensland Department of Mines and Energy previously used a range of 12–40 t/ha/year as a target erosion rate for rehabilitated mine sites [2, 3], which was also impacted by the fact that it should be manageable to levels that do not compromise post-mining land use.

It is also worth mentioning that there are a few Australian studies that have attempted to quantify erosion rates. Lu et al. [4] utilized spatial modeling methods (remotely sensed data) to predict the sheetwash (interrill) and rill erosion all over the Australian continent. They estimated that the average erosion rate is 4.1 ton/ha/year over the Australian continent; however, they stated that "Soil erosion is naturally highly variable. This needs to be recognized when comparing current rates of erosion from one place to another and when the erosion control policies are set. It should be expressed in relation to spatially variability, rather than referring to absolute rates alone or using a single benchmark applied across diverse landscapes."

Since it seems difficult to agree on a specific value for erosion rate threshold, the determination of this threshold value must be done for each mining site independently, to make it conceivable to achieve the required erosion stability; and that which decision makers and governmental regulators work on this matter will accept. The concept that the soil erosion rate threshold for specific mining site should be equal or close to the erosion rate values of the surrounding areas in similar climatic and environmental conditions deserves support and should be applied; the value of this threshold should be also manageable to levels that do not compromise post-mining land use agreed upon with the local community and by the PRC plan.

Rainfall simulators are distinguished research tools that enable us to measure erosion rates for any mine site and its surrounding area in the laboratory or in the field with high accuracy and efficiency; the erodibility values of soil and materials that will be used as land cover can also be measured accurately. Therefore, the rainfall simulators can provide us with some important data necessary for analysis and erosion modeling processes to help us design the best land cover system that could achieve long-term erosional stability.
