Environmental Kuznets Curve Analysis of the Economic Development and Nonpoint Source Pollution in the Ningxia Yellow River Irrigation Districts in China

Table 3

Relationship between living-sewage pollution load and per capita income.

Curve type	Constant term	First-order coefficient	Second-order coefficient	Third-order coefficient	𝑅^2	𝐹
Quadratic coefficient	9.5409 (10.48)***	−0.2893 (−1.18)	0.0221 (1.35)	—	0.64	14.40
Three coefficients	22.6854 (1.44)	−5.5748 (−0.88)	0.7280 (0.86)	−0.0313 (−0.84)	0.66	9.65

Note. ***, **, and *, respectively, represent significance levels in 1%, 5%, and 10%. The 𝑡-test results are written in brackets.