Designing a rectangular water tank resting on the ground involves considering several factors such as the tank's capacity, structural integrity, stability, and the soil conditions where it will be placed. 

Here's a general overview of the design considerations:

Tank Capacity and Dimensions: Determine the required capacity of the water tank based on the intended usage and demand. Then, decide on the dimensions of the tank, considering factors such as available space, ease of construction, and transportation limitations.

Material Selection: Choose a suitable material for constructing the tank based on factors such as cost, durability, and compatibility with the stored water. Common materials for water tanks include concrete, steel, fiberglass, and plastic (polyethylene).

Structural Design: The structural design of the tank should ensure that it can withstand the loads imposed by the stored water, as well as any additional loads such as wind and seismic forces. Consider factors such as wall thickness, reinforcement (if applicable), and the design of support elements such as beams and columns.

Foundation Design: The tank will be resting directly on the ground, so it's important to ensure a stable foundation. Conduct a geotechnical investigation to assess soil conditions and determine the appropriate foundation type. Depending on soil characteristics, options may include a shallow concrete pad, a reinforced concrete slab, or spread footings.

Waterproofing and Sealing: Ensure that the tank is properly waterproofed to prevent leakage and contamination of the stored water. This may involve applying a suitable sealant or membrane to the interior surfaces of the tank and providing adequate drainage to prevent water buildup around the tank base.

Access and Maintenance: Design the tank with appropriate access points for inspection, cleaning, and maintenance. This may include access hatches, manholes, and fittings for connecting piping and accessories.

Overflow and Ventilation: Incorporate provisions for overflow and ventilation to prevent overfilling of the tank and the buildup of pressure or gases inside. Overflow pipes should discharge safely away from the tank foundation to prevent erosion.

Compliance with Regulations: Ensure that the design and construction of the water tank comply with relevant codes, standards, and regulations governing water storage facilities in your locality. This may include requirements related to structural design, materials, safety, and sanitation.

Construction and Quality Control: During construction, implement quality control measures to verify that the tank is built according to the design specifications and standards. This may involve testing materials, inspecting construction processes, and monitoring workmanship.

By carefully considering these design factors and adhering to best practices in water tank construction, you can ensure the successful and reliable operation of the rectangular water tank resting on the ground. Consulting with structural engineers, geotechnical engineers, and regulatory authorities can provide valuable guidance and expertise throughout the design and construction process.