## 3 Materials and Methods
### Process-Based or Conceptual Models
Topsoil carbon undergoes various transformations mainly as dissolved organic carbon (DOC) and particulate organic carbon (POC). The latter has a vivid role in numerous flocculation and adsorption phenomena affecting soil physicochemical properties. DOC represents the soluble organic part and holds a crucial role in soil transformation via microorganisms' uptake availability, the high soil mobility/leaching, biochemical transformation by means of metal ion/organic chemical binding and toxicity phenomena through biosorption. Ref. [59] developed and introduced a modified TRIPLEX-DOC model focused on leaching, sorption, DOC and biodegradation phenomena [60] to simulate DOC dynamics in monsoon forest ecosystems since it does not incorporate itself as a component simulating responses of DOC concentrations and fluxes to the atmospheric N deposition. By means of TRIPLEX-DOC model improvements, a more profound biogeochemical model was incorporated to predict more accurately DOC fluxes in forest ecosystems in the monsoon regions which differentiate them in terms of vegetation and NPP. Furthermore, the modified model predicts more accurately regional carbon cycle in seasonal patterns and assesses more safely soil DOC fluxes in local ecosystems. Nitrate leaching occurs in croplands where synthetic and animal biowaste-nitrogen-based fertilizers are applied, which affect vulnerable aquifers (e.g., karsts) from nitrate pollution. The soil and water assessment tool (SWAT) [61], developed by the US Department of Agriculture (USDA) and agricultural research service (ARS), is widely exploited to simulate the hydrological cycle, plant growth, and chemical leaching in numerous watersheds and cultivated soils. SWAT (CATCH) is a distributed, semi-empirical [62], physically based hydrologic model (DPBHM), which incorporates the variable storage coefficient (VSC) method. The latter is a kinematic wave method, which means that the streamflow equation is simplified by assuming that the friction slope is approximately equal to the slope of the, e.g., watershed channel. The proper use of SWAT simulation corrects the initial assessment of certain indices, e.g., dendritic connectivity index (DCI), a measure of longitudinal connectivity of aquatic ecosystem status, which outlines that the temporal resolution of river water routing (time steps) plays a dominant role in the final assessments. It is noteworthy that time steps are dependent on actual watershed size and configuration [63]. SWAT employs the vegetative filter strip MODel (VFSMOD), which is an empirical model [64] that calculates sediment reduction. It simulates crop yields and nitrate leaching under a variety of nutrient and irrigation management practices as a well-tested prediction tool to improve management practices upon safeguarding high groundwater quality towards agricultural sustainability. SWAT is combined easily with sequential uncertainty fitting (SUFI-2), uncertainty procedures (SWAT-CUP 2012), and the Nash-Sutcliffe model efficiency (NSE) validation test [65]. SWAT is modified by coupling with shuffled frog leaping algorithms and a farm-level economic model and cost estimator (FEM). The model outcome (MOSFLA) demonstrates powerful convergence and optimization ability [66]. The SWAT tool predicts and quantifies nitrate leaching pathways and prevents groundwater nitrate pollution, which threatens drinking water resources and unbalances ecosystems. Nitrate pollution assessment changes fertilizing practices, alters the crops and prioritizes new sustainable agricultural methodologies and management practices in agribusiness. The crucial nitrate pollution drivers are the type of the soil, climate variability, crops irrigation, and fertilization practices over cropland [65].