There is an urgent need for inexpensive methods to characterize soil hydraulic properties accurately. For example, it is crucial for effective management and irrigation strategies in arid regions where water scarcity and land degradation due to water erosion and salinity are pressing concerns. Similarly, in temperate regions where landslides have increased due to intensified rainfall, precise measurement of rainfall infiltration into soils is needed for accurate landslide forecasting. This study introduces an affordable optical water level meter and assesses its effectiveness in determining field-saturated hydraulic conductivity (Kfs) using the twice-repeated falling-head method. Kfs values were derived using Excel's solver function via a non-linear least-squares approach. Numerical simulations were conducted under various conditions, including different soil types, water level ranges (H), cylinder radii (r), and insertion depths (d), utilizing HYDRUS (2D/3D) Ver. 3.01. The relationship between the dimensionless shape factor (Gt) and the (d/r) ratio was examined for the following water level measurement ranges: 5 ≤ H ≤ 20 cm, 5 ≤ H ≤ 25 cm, and 15 ≤ H ≤ 25 cm. The findings emphasize the importance of a self-recording meter to accurately measure water levels in soils with extremely high permeability, such as sandy soils, or very low permeability, such as clayey soils.

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