## 1 Introduction  
Land degradation affects ecosystem productivity and threatens its capacity to sustain human, livestock, and wildlife population specially in dryland environments. Drylands that are susceptible to desertification occupy 39.7% (~5.2 billion ha) of the global terrestrial ecosystems (~13 billion ha) [1; 2]. Of this, sever land degradation is prevalent in over 10-20% of the dryland ecosystems [1; 2; 3]. For these reasons, land degradation in dryland ecosystems is recognized as one of the major environmental and socioeconomic challenges that can alter ecosystem services and human wellbeing [4; 5; 6; 7]. Thus, understanding the rate, expansion, and severity of drylands degradation has received (and will continue to receive) considerable attention, due to their pivotal role in food production and water availability for more than 2 billion people in the world [3; 8; 9; 10; 11]. The main causes of drylands degradation are principally associated with population growth, overgrazing, inappropriate land and water use practices, and climate change impacts [1; 11; 12]. As a result, noticeable and persistent loss of vegetation cover and biomass productivity, reduction in forage and crop production, and decline in carrying capacity of rangelands are becoming common in these ecosystem [3; 13; 14]. Future predictions showed that climate change impacts (i.e., increase in surface air temperature and evapotranspiration, and decrease in precipitation) are expected to worsen poverty and inequality in developing countries [15; 16]. Changes in above-ground biomass in drylands on which forage production and other life securing ecosystem services depend on can be measured by the net primary production (NPP) [13; 17]. Several studies used temporal changes in NPP as an indicator of land degradation [2; 18; 19; 20]. Nevertheless, the measurements and estimation of land degradation have been characterized by arbitrary assumptions, qualitative and inconsistent judgment, unreliable and spurious estimations [7; 21; 22]. Several studies have been criticized for underestimating the extent and severity of rangeland degradation [3; 8; 23]. The main reasons being the climate of dryland ecosystems (i.e., low annual precipitation and its increased interannual variability) [19; 24], the successive occurrence of degradation for many decades at regional or continental scales [20], and the lack of objective measurements that quantify the extent and severity of all forms of degradation [4]. These reasons further complicate the identification of changes in dryland productivity either due to human activities (grazing and cropping) or natural variability [13; 17]. Hence, the assessment of drylands degradation requires techniques that encompass spatial and temporal properties that strongly adhere to the measurement principles of repetitiveness, objectivity, and consistency [21; 25]. In this regard, earth observations (OE) proved to be the only feasible means for long time and large-scale monitoring of vegetation productivity [8; 9; 18; 24]. Vegetation indices (VIs) such as the normalized difference vegetation index (NDVI) have been used as proxy to detect and quantify long-term land degradation in drylands [6; 7; 11; 17; 18; 21; 25]. Two main categories of methodology have been employed to assess land degradation in drylands [7; 22]--the first one observes changes in relationships between climate variables (i.e., precipitation and temperature) and VIs (i.e., measure of vegetation greenness and productivity) [8; 22; 23]. The second category analyzes vegetation phenology to detect structural change caused by processes such as deforestation and long-term trends [22; 24]. Within each category, various methods have been applied. For example, the methods in the first category aim to control the influence of precipitation and/or temperature trends to detect permanent degradation [7; 23].